DARWIN,  AND  AFTER  DARWIN 
m 

POST-DARWINIAN   QUESTIONS 
ISOLATION    AND    PHYSIOLOGICAL   SELECTION 


BY  THE  SAME  AUTHOR. 


DARWIN,  AND  AFTER  DARWIN.  An  Exposition  of  the 
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3.  POST-DARWINIAN  QUESTIONS.    ISOLATION  AND  PHY- 

SIOLOGICAL SELECTION.    Edited  by  Prof.  C.  Lloyd 
Morgan.     With  portrait  of  Mr.  J.  T.  Gulick.    181 
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THOUGHTS  ON  RELIGION.  Edited  by  Charles  Gore,  M.A., 
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THE  OPEN  COURT  PUBLISHING  COMPANY, 

324  DEARBORN  STREET,  CHICAGO. 


DARWIN,  AND  AFTER  DARWIN 


AM  EXPOSITION  OF  THE  DARWINIAN  THEORY 

AND  A  DISCUSSION  OF 
POST-DARWINIAN  QUESTIONS 


BY  THE  LATE 

GEORGE  JOHN  ROMANES,  M.A.,  LL.D.,  F.R.S. 

Honorary  Fellow  of  Gon-ville  and  Caius  College,  Cambridge 


III 

POST-DARWINIAN  QUESTIONS 

ISOLATION 
AND  PHYSIOLOGICAL  SELECTION 


Chicago 

THE   OPEN   COURT   PUBLISHING   COMPANY 

1897 

93423 


CHAPTER  I.   COPYRIGHTED  BY 

THE  OPEN  COURT  PUBLISHING  CO. 


R.  R.  DONNELLEY  *  SONS  CO.,  CHICAGO 


3GG 


v.  3 


PREFACE 


OF  the  six  chapters  which  constitute  this  con- 
cluding volume  of  G.  J.  Romanes'  Darwin,  and  after 
Darwin,  three,  the  first  two  and  the  last,  were  in 
type  at  the  time  of  his  death.  I  have  not  considered 
myself  at  liberty  to  make  any  alterations  of  moment 
in  these  chapters.  For  the  selection  and  arrange- 
ment of  all  that  is  contained  in  the  other  three 
chapters  I  am  wholly  responsible. 

Two  long  controversial  Appendices  have  been 
omitted.  Those  marked  A  and  B  remain  in  accord- 
ance with  the  author's  expressed  injunctions.  In 
a  third,  marked  C,  a  few  passages  from  the  author's 
note-books  or  MSS.  have  been  printed. 

The  portrait  of  the  Rev.  J.  Gulick,  which  forms  the 
frontispiece,  was  prepared  for  this  volume  before  the 
author's  death.  Mr.  Gulick's  chief  contributions  to 
the  theory  of  physiological  selection  are  to  be  found 
in  the  Linnean  Society's  Journal  (Zoology,  vols.  xx 


vi  Preface. 

and  xxiii),  and  in  four  letters  to  Nature  (vol.  xli. 
p.  536 ;  vol.  xlii.  pp.  28  and  369 ;  and  vol.  xliv. 
p.  29). 

I  have  to  thank  Mr.  Francis  Galton,  D.C.L.,  F.R.S. 
and  Mr.  F.  Howard  Collins  for  valuable  assistance 
generously  rendered  for  the  sake  of  one  whom  all 
who  knew  him  held  dear.  For  he  was,  if  I  may 
echo  the  words  of  Huxley,  "a  friend  endeared  to 
me,  as  to  so  many  others,  by  his  kindly  nature,  and 
justly  valued  by  all  his  colleagues  for  his  powers 
of  investigation  and  his  zeal  for  the  advancement  of 

science." 

C.  LLOYD  MORGAN. 

BRISTOL,  May  1897. 


CONTENTS 


CHAPTER  L 

MOB 

ISOLATION i 

CHAPTER   U. 
ISOLATION  (continued} 38 

CHAPTER   III. 
PHYSIOLOGICAL  SELECTION 41 

CHAPTER   IV. 
EVIDENCES  OF  PHYSIOLOGICAL  SELECTION  ...     62 

CHAPTER  V. 
FURTHER  EVIDENCES  OF  PHYSIOLOGICAL  SELECTION  .      81 

CHAPTER  VI. 

A  BRIEF  HISTORY  OF   ISOLATION  AS  A  FACTOR  IN 
ORGANIC  EVOLUTION 101 

GENERAL  CONCLUSIONS 144 


APPENDIX  A.   MR.  GULICK'S  CRITICISM  OF  MR.  WALLACE'S 
VIEWS  ON  PHYSIOLOGICAL  SELECTION    .        .       -151 


viii  Contents. 

PACK 

APPENDIX  B.  AN  EXAMINATION  BY  MR.  FLETCHER 
MOULTON  OF  MR.  WALLACE'S  CALCULATION  TOUCH- 
ING THE  POSSIBILITY  OF  PHYSIOLOGICAL  SELECTION 
EVER  ACTING  ALONE 157 

APPENDIX  C.  SOME  EXTRACTS  FROM  THE  AUTHOR'S 
NOTE-BOOKS 169 


ISOLATION 


DARWIN,  AND  AFTER  DARWIN. 

CHAPTER  I. 
ISOLATION. 

THIS  treatise  will  now  draw  to  a  close  by  considering 
what,  in  my  opinion,  is  one  of  the  most  important 
principles  that  are  concerned  in  the  process  of  organic 
evolution — namely,  Isolation.  I  say  in  my  opinion 
such  is  the  case,  because,  although  the  importance  of 
isolation  is  more  or  less  recognized  by  every  naturalist, 
I  know  of  only  one  other  who  has  perceived  all  that 
the  principle  involves.  This  naturalist  is  the  Rev.  J. 
Gulick,  and  to  his  essays  on  the  subject  I  attribute 
a  higher  value  than  t  o  any  other  work  in  the  field  of 
Darwinian  thought  since  the  date  of  Darwin's  death l. 
For  it  is  now  my  matured  conviction  that  a  new  point 
of  departure  has  here  been  taken  in  the  philosophy  of 
Darwinism,  and  one  which  opens  up  new  territories 
for  scientific  exploration  of  an  endlessly  wide  and 
varied  character.  Indeed  I  believe,  with  Mr.  Gulick, 

1  It  will  be  remembered  that  I  regard  Weismann's  theory  of  heredity, 
with  all  its  deductive  consequences,  as  still  subjudice. 
III.  B 


2  Darwin,  and  after  Darwin. 

that  in  the  principle  of  Isolation  we  have  a  principle 
so  fundamental  and  so  universal,  that  even  the  great 
principle  of  Natural  Selection  lies  less  deep,  and 
pervades  a  region  of  smaller  extent.  Equalled  only 
in  its  importance  by  the  two  basal  principles  of 
Heredity  and  Variation,  this  principle  of  Isolation 
constitutes  the  third  pillar  of  a  tripod  on  which  is 
reared  the  whole  superstructure  of  organic  evolution. 

By  isolation  I  mean  simply  the  prevention  of  inter- 
crossing between  a  separated  section  of  a  species  or 
kind  and  the  rest  of  that  species  or  kind.  Whether 
such  a  separation  be  due  to  geographical  barriers,  to 
migration,  or  to  any  other  state  of  matters  leading 
to  exclusive  breeding  within  the  separated  group. 
I  shall  indifferently  employ  the  term  isolation  for  the 
purpose  of  designating  what  in  all  cases  is  the  same 
result — namely,  a  prevention  of  intercrossing  between 
A  and  B,  where  A  is  the  separated  portion  and  B  the 
rest  of  the  species  or  kind. 

The  importance  of  isolation  as  against  dissimilar 
forms  has  always  been  fully  appreciated  by  breeders 
fanciers,  horticulturists,  &c.,  who  are  therefore  most 
careful  to  prevent  their  pedigree  productions  from 
intercrossing  with  any  other  stock.  Isolation  is  indeed, 
as  Darwin  has  observed,  "  the  corner-stone  of  the 
breeder's  art."  And  similarly  with  plants  and  animals 
in  a  state  of  nature :  unless  intercrossing  with  allied 
(i.e.  dissimilar)  forms  is  prevented,  the  principle  of 
heredity  is  bound  to  work  for  uniformity,  by  blend- 
ing the  dissimilar  types  in  one :  only  when  there  is 
exclusive  breeding  of  similarly  modified  forms  can  the 
principle  of  heredity  work  in  the  direction  of  change 
— i.e.  of  evolution. 


Isolation.  3 

Now,  the  forms  of  isolation — or  the  conditions 
which  may  lead  to  exclusive  breeding — are  manifold. 
One  of  the  most  important,  as  well  as  the  most  obvious, 
is  geographical  isolation ;  and  no  one  questions  that 
this  has  been  an  important  factor  in  the  process  of 
evolution,  although  opinions  still  vary  greatly  as  to 
the  degree  of  its  importance  in  this  respect.  At  one 
end  of  the  series  we  may  place  the  opinion  of  Mr. 
Wallace,  who  denies  that  any  of  what  may  be  termed 
the  evolutionary  effect  of  geographical  isolation  is  due 
to  "  influence  exerted  by  isolation  per  se."  This  effect, 
he  says,  is  to  be  ascribed  exclusively  to  the  fact  that 
a  geographically  isolated  portion  of  a  species  must 
always  encounter  a  change  of  environment,  and  there- 
fore a  new  set  of  conditions  necessitating  a  new  set  of 
adaptations  at  the  hands  of  natural  selection 1.  At 
the  other  end  of  the  series  we  must  place  the  opinion 
of  Moritz  Wagner,  who  many  years  ago  published 
a  masterly  essay  2,  the  object  of  which  was  to  prove 
that,  in  the  absence  of  geographical  isolation  (including 
migration),  natural  selection  would  be  powerless  to 
effect  any  change  of  specific  type.  For,  he  argued, 
the  initial  variations  on  which  the  action  of  this 
principle  depends  would  otherwise  be  inevitably 
swamped  by  free  intercrossing.  Wagner  adduced 
a  large  number  of  interesting  facts  in  support  of  this 
opinion ;  but  although  he  thus  succeeded  in  en- 
forcing the  truth  that  geographical  isolation  is  an 
important  aid  to  organic  evolution,  he  failed  to  establish 
his  conclusion  that  it  is  an  indispensable  condition. 

1  Darwinism,  p.  150. 

1  The  Darwinian  Theory,  and  the  Law  of  Migration  (Eng.  Trans., 
Stanford,  London,  1873). 

B   2 


4  Darwin,  and  after  Darwin. 

Nevertheless  he  may  have  been  right — and,  as  I  shall 
presently  show,  I  believe  he  was  right — in  his  funda- 
mental premiss,  that  in  the  presence  of  free  inter- 
crossing natural  selection  would  be  powerless  to  effect 
divergent  evolution.  Where  he  went  wrong  was  in 
not  perceiving  that  geographical  isolation  is  not  the 
only  form  of  isolation.  Had  it  occurred  to  him  that 
there  may  be  other  forms  quite  as  effectual  for  the 
prevention  of  free  intercrossing,  his  essay  could  hardly 
have  failed  to  mark  an  epoch  in  the  history  of  Dar- 
winism. But,  on  account  of  this  oversight,  he  really 
weakened  his  main  contention,  namely,  that  in  the 
presence  of  free  intercrossing  natural  selection  must 
be  powerless  to  effect  divergent  evolution.  This  main 
contention  I  am  now  about  to  re-argue.  At  present, 
therefore,  we  have  only  to  observe  that  Wagner  did  it 
much  more  harm  than  good  by  neglecting  to  perceive 
that  free  intercrossing  may  be  prevented  in  many  other 
ways  besides  by  migration,  and  by  the  intervention  of 
geographical  barriers. 

In  order  that  we  may  set  out  with  clearer  views 
upon  this  matter,  I  will  make  one  or  two  preliminary 
remarks  on  the  more  general  facts  of  isolation  as  these 
are  found  to  occur  in  nature. 

In  the  first  place,  it  is  obvious  that  isolation 
admits  of  degrees :  it  may  be  either  total  or  partial ; 
and,  if  partial,  may  occur  in  numberless  grades  of 
efficiency.  This  is  so  manifest  that  I  need  not  wait 
to  give  illustrations.  But  now,  in  the  second  place, 
there  is  another  general  fact  appertaining  to  isolation 
which  is  not  so  manifest,  and  a  clear  appreciation 
of  which  is  so  essential  to  any  adequate  considera- 
tion of  the  subject,  that  I  believe  the  reason  why 


Isolation.  5 

evolutionists  have  hitherto  failed  to  perceive  the  full 
importance  of  isolation,  is  because  they  have  failed 
to  perceive  the  distinction  which  has  now  to  be  pointed 
out.  The  distinction  is,  that  isolation  may  be  either 
discriminate  or  indiscriminate.  If  it  be  discriminate, 
the  isolation  has  reference  to  the  resemblance  of  the 
separated  individuals  to  one  another;  if  it  be  indis- 
criminate, it  has  no  such  reference.  For  example,  if 
a  shepherd  divides  a  flock  of  sheep  without  regard  to 
their  characters,  he  is  isolating  one  section  from  the 
other  indiscriminately ;  but  if  he  places  all  the  white 
sheep  in  one  field,  and  all  the  black  sheep  in  another 
field,  he  is  isolating  one  section  from  the  other 
discriminately.  Or,  if  geological  subsidence  divides 
a  species  into  two  parts,  the  isolation  will  be  indis- 
criminate ;  but  if  the  separation  be  due  to  one  of 
the  sections  developing,  for  example,  a  change  of 
instinct  determining  migration  to  another  area,  or 
occupation  of  a  different  habitat  on  the  same  area, 
then  the  isolation  will  be  discriminate,  so  far  as  the 
resemblance  of  instinct  is  concerned. 

With  the  exception  of  Mr.  Gulick,  I  cannot  find 
that  any  other  writer  has  hitherto  stated  this 
supremely  important  distinction  between  isolation  as 
discriminate  and  indiscriminate.  But  he  has  fully 
as  well  as  independently  stated  it,  and  shown  in 
a  masterly  way  its  far-reaching  consequences.  Indis- 
criminate isolation  he  calls  Separate  Breeding,  while 
discriminate  isolation  he  calls  Segregate  Breeding. 
For  the  sake  however,  of  securing  more  descriptive 
terms,  I  will  coin  the  words  Apogamy  and  Homogamy. 
Apogamy,  of  course,  answers  to  indiscriminate  isola- 
tion, or  separate  breeding.  Homogamy,  on  the  other 


6  Darwin,  and  after  Darwin. 

hand,  answers  to  discriminate  isolation,  or  segregate 
breeding:  only  individuals  belonging  to  the  same 
variety  or  kind  are  allowed  to  propagate.  Isolation, 
then,  is  a  genus,  of  which  Apogamy  and  Homogamy 
are  species1. 

Now,  in  order  to  appreciate  the  unsurpassed  im- 
portance of  isolation  as  one  of  the  three  basal 
principles  of  organic  evolution,  let  us  begin  by 
considering  the  discriminate  species  of  it,  or  Homo- 
gamy. 

To  state  the  case  in  the  most  general  terms,  we 
may  say  that  if  the  other  two  basal  principles  are 
given  in  heredity  and  variability,  the  whole  theory 
of  organic  evolution  becomes  neither  more  nor  less 
than  a  theory  of  homogamy — that  is,  a  theory  of 
the  causes  which  lead  to  discriminate  isolation,  or 
the  breeding  of  like  with  like  to  the  exclusion  of 
unlike.  For  the  more  we  believe  in  heredity  and 
variability  as  basal  principles  of  organic  evolution, 
the  stronger  must  become  our  persuasion  that  dis- 
criminate breeding  leads  to  divergence  of  type,  while 
indiscriminate  breeding  leads  to  uniformity.  This, 
in  fact,  is  securely  based  on  what  we  know  from  the 
experience  supplied  by  artificial  selection,  which  con- 

1  I  may  here  most  conveniently  define  the  senses  in  which  all  the 
following  terms  will  be  used  throughout  the  present  discussion  : — Species 
of  isolation  are,  as  above  stated,  homogamy  and  apogamy,  or  isolation 
as  discriminate  and  indiscriminate.  Forms  of  isolation  are  modes  of 
isolation,  such  as  the  geographical,  the  sexual,  the  instinctive,  or  any 
other  of  the  numerous  means  whereby  isolation  of  either  species  may  be 
secured.  Cases  of  isolation  are  the  instances  in  which  any  of  the  forms 
of  isolation  may  be  at  work  :  thus,  if  a  group  of  n  intergenerants  be 
segregated  into  five  groups,  a,  b,  c,  J,  e,  then,  before  the  segregation  there 
would  have  been  one  case  of  isolation,  but  after  the  segregation  there 
would  be  five  such  cases. 


Isolation.  7 

sists  in  the  intentional  mating  of  like  with  like  to  the 
exclusion  of  unlike. 

The  point,  then,  which  in  the  first  instance  must  be 
firmly  fastened  in  our  minds  is  this  : — so  long  as  there 
is  free  intercrossing,  heredity  cancels  variability,  and 
makes  in  favour  of  fixity  of  type.  Only  when  as- 
sisted by  some  form  of  discriminate  isolation,  which 
determines  the  exclusive  breeding  of  like  with  like, 
can  heredity  make  in  favour  of  change  of  type,  or 
lead  to  what  we  understand  by  organic  evolution. 

Now  the  forms  of  discriminate  isolation,  or  homo- 
gamy,  are  very  numerous.  When,  for  example,  any 
section  of  a  species  adopts  somewhat  different  habits 
of  life,  or  occupies  a  somewhat  different  station  in 
the  economy  of  nature,  homogamy  arises  within  that 
section.  There  are  forms  of  homogamy  on  which 
Darwin  has  laid  great  stress,  as  we  shall  presently 
find.  Again,  when  for  these  or  any  other  reasons  a 
section  of  a  species  becomes  in  any  small  degree 
modified  as  to  form  or  colour,  if  the  species  happens 
to  be  one  where  any  psychological  preference  in 
pairing  can  be  exercised — as  is  very  generally  the 
case  among  the  higher  animals — exclusive  breeding 
is  apt  to  ensue  as  a  result  of  such  preference  ;  for 
there  is  abundant  evidence  to  show  that,  both  in  birds 
and  mammals,  sexual  selection  is  usually  opposed  to 
the  intercrossing  of  dissimilar  varieties.  Once  more, 
in  the  case  of  plants,  intercrossing  of  dissimilar 
varieties  may  be  prevented  by  any  slight  difference  in 
their  seasons  of  flowering,  of  topographical  stations, 
or  even,  in  the  case  of  flowers  which  depend  on 
insects  for  their  fertilization,  by  differences  in  the 
instincts  and  preferences  of  their  visitors. 


8  Darwin,  and  after  Darwin. 

But,  without  at  present  going  into  detail  with 
regard  to  these  different  forms  of  discriminate  isolation, 
there  are  still  two  others,  both  of  which  are  of  much 
greater  importance  than  any  that  I  have  hitherto 
named.  Indeed,  these  two  forms  are  of  such  im- 
measurable importance,  that  were  it  not  for  their 
virtually  ubiquitous  operation,  the  process  of  organic 
evolution  could  never  have  begun,  nor,  having  begun, 
continued. 

The  first  of  these  two  forms  is  sexual  incompati- 
bility— either  partial  or  absolute — between  different 
taxonomic  groups.  If  all  hares  and  rabbits,  for 
example,  were  as  fertile  with  one  another  as  they 
are  within  their  own  respective  species,  there  can  be 
no  doubt  that  sooner  or  later,  and  on  common  areas, 
the  two  types  would  fuse  into  one.  And  similarly, 
if  the  bar  of  sterility  could  be  thrown  down  as 
between  all  the  species  of  a  genus,  or  all  the  genera  of 
a  family,  not  otherwise  prevented  from  intercrossing, 
in  time  all  such  species,  or  all  such  genera,  would 
become  blended  into  a  single  type.  As  a  matter 
of  fact,  complete  fertility,  both  of  first  crosses  and 
of  their  resulting  hybrids,  is  rare,  even  as  between 
species  of  the  same  genus  ;  while  as  between  genera 
of  the  same  family  complete  fertility  does  not  appear 
ever  to  occur ;  and,  of  course,  the  same  applies  to 
all  the  higher  taxonomic  divisions.  On  the  other 
hand,  some  degree  of  infertility  is  not  unusual  as 
between  different  varieties  of  the  same  species ;  and, 
wherever  this  is  the  case,  it  must  clearly  aid  the  further 
differentiation  of  those  varieties.  It  will  be  my 
endeavour  to  show  that  in  this  latter  connexion 
sexual  incompatibility  must  be  held  to  have  taken 


Isolation.  9 

an  immensely  important  part  in  the  differentiation 
of  varieties  into  species.  But  meanwhile  we  have 
only  to  observe  that  wherever  such  incompatibility  is 
concerned,  it  is  to  be  regarded  as  an  isolating  agency 
of  the  very  first  importance.  And  as  it  is  of  a 
character  purely  physiological,  I  have  assigned  to  it 
the  name  Physiological  Isolation ;  while  for  the  par- 
ticular case  where  this  general  principle  is  concerned 
in  the  origination  of  specific  types,  I  have  reserved 
the  name  Physiological  Selection. 

The  other  most  important  form  of  discriminate 
isolation  to  which  I  have  alluded  is  Natural  Selection. 
To  some  evolutionists  it  has  seemed  paradoxical 
thus  to  regard  natural  selection  as  a  form  of  isola- 
tion ;  but  a  little  thought  will  suffice  to  show  that 
such  is  really  the  most  accurate  way  of  regarding  it. 
For,  as  Mr.  Gulick  says,  "Natural  selection  is  the 
exclusive  breeding  of  those  better  adapted  to  the  en- 
vironment :  ...  it  is  a  process  in  which  the  fittest  are 
prevented  from  crossing  with  the  less  fitted,  by  the 
exclusion  of  the  less  fitted/'  Therefore  it  is,  strictly 
and  accurately,  a  mode  of  isolation,  where  the 
isolation  has  reference  to  adaptation,  and  is  secured 
in  the  most  effectual  of  possible  ways — i.  e.  by  the 
destruction  of  all  individuals  whose  intercrossing  would 
interfere  with  the  isolation.  Indeed,  the  very  term 
"  natural  selection  "  shows  that  the  principle  is  tacitly 
understood  to  be  one  of  isolation,  because  this  name 
was  assigned  to  the  principle  by  Darwin  for  the 
express  purpose  of  marking  the  analogy  that  obtains 
between  it  and  the  intentional  isolation  which  is 
practised  by  breeders,  fanciers,  and  horticulturists. 
The  only  difference  between  "  natural  selection  "  and 


io  Darwin,  and  after  Darwin. 

"  artificial  selection  "  consists  in  this— that  under  the 
former  process  the  excluded  individuals  must  neces- 
sarily perish,  while  under  the  latter  they  need  not  do 
so.  But  clearly  this  difference  is  accidental :  it  is  in 
no  way  essential  to  the  process  considered  as  a  process 
of  discriminate  isolation.  For,  as  far  as  homogamous 
breeding  is  concerned,  it  can  matter  nothing  whether 
the  exclusion  of  the  dissimilar  individuals  is  effected 
by  separation  or  by  death. 

Natural  selection,  then,  is  thus  unquestionably 
a  form  of  isolation  of  the  discriminate  kind  ;  and 
therefore,  notwithstanding  its  unique  importance  in 
certain  respects,  considered  as  a  principle  of  organic 
evolution  it  is  less  fundamental — and  also  less  ex- 
tensive— than  the  principle  of  isolation  in  general.  In 
other  words,  it  is  but  a  part  of  a  much  larger  whole. 
It  is  but  a  particular  form  of  a  general  principle, 
which,  as  just  shown,  presents  many  other  forms,  not 
only  of  the  discriminate,  but  likewise  of  the  indiscri- 
minate kind.  Or,  reverting  to  the  terminology  of 
logic,  it  is  a  sub-species  of  the  species  Homogamy, 
which  in  its  turn  is  but  a  constituent  part  of  the 
genus  Isolation. 

So  much  then  for  homogamy,  or  isolation  of  the 
discriminate  order.  Passing  on  now  to  apogamy,  or 
isolation  of  the  indiscriminate  kind,  we  may  well  be 
disposed,  at  first  sight,  to  conclude  that  this  kind  of 
isolation  can  count  for  nothing  in  the  process  of  evo- 
lution. For  if  the  fundamental  importance  of  isola- 
tion in  the  production  of  organic  forms  be  due  to  its 
segregation  of  like  with  like,  does  it  not  follow 
that  any  form  of  isolation  which  is  indiscriminate 
must  fail  to  supply  the  very  condition  on  which  all 


Isolation.  n 

the  forms  of  discriminate  isolation  depend  for  their 
efficacy  in  the  causing  of  organic  evolution  ?  Or,  to 
return  to  our  concrete  example,  is  it  not  self-evident 
that  the  farmer  who  separated  his  stock  into  two 
or  more  parts  indiscriminately,  would  not  effect  any 
more  change  in  his  stock  than  if  he  had  left  them 
all  to  breed  together  ? 

Well,  although  at  first  sight  this  seems  self-evident, 
it  is  in  fact  untrue.  For,  unless  the  individuals  which 
are  indiscriminately  isolated  happen  to  be  a  very 
large  number,  sooner  or  later  their  progeny  will  come 
to  differ  from  that  of  the  parent  type,  or  unisolated 
portion  of  the  previous  stock.  And.  of  course,  as 
soon  as  this  change  of  type  begins,  the  isolation 
ceases  to  be  indiscriminate :  the  previous  apogamy 
has  been  converted  into  homogamy,  with  the  usual 
result  of  causing  a  divergence  of  type.  The  reason 
why  progeny  of  an  indiscriminately  isolated  section 
of  an  originally  uniform  stock — e.g.  of  a  species — will 
eventually  deviate  from  the  original  type  is,  to  quote 
Mr.  Gulick.  as  follows  : — "  No  two  portions  of  a  species 
possess  exactly  the  same  average  character,  and, 
therefore,  the  initial  differences  are  for  ever  reacting 
on  the  environment  and  on  each  other  in  such  a  way 
as  to  ensure  increasing  divergence  as  long  as  the 
individuals  of  the  two  groups  are  kept  from  inter- 
generating1."  Or,  as  I  stated  this  principle  in  my 
essay  on  Physiological  Selection,  published  but  a  short 
time  before  Mr.  Gulick's  invaluable  contributions  to 
these  topics : — 

As  a  matter  of  fact,  we  find  that  no  one  individual  "is  like 

1  Divergent  Evolution  through  Cumulative  Segregation  (Zool.  Journal, 
Linn.  Soc.,  vol.  xx.  pp.  189-374). 


12          Darwin,  and  after  Darwin. 

another  all  in  all " ;  which  is  another  way  of  saying  that  a 
specific  type  may  be  regarded  as  the  average  mean  of  all  its  in- 
dividual variations,  any  considerable  departure  from  this  average 
being,  however,  checked  by  intercrossing.  .  .  .  Consequently,  if 
from  any  cause  a  section  of  a  species  is  prevented  from  inter- 
crossing with  the  rest  of  its  species,  we  might  expect  that  new 
varieties  should  arise  within  that  section,  and  that  in  time  these 
varieties  should  pass  into  new  species.  And  this  is  just  what 
we  do  find1. 

The  name  which  I  gave  to  this  cause  of  specific 
change  was  Independent  Variability,  or  variability  in 
the  absence  of  overwhelming  intercrossing.  But  it 
now  appears  to  me  that  this  cause  is  really  identi- 
cal with  that  which  was  previously  enunciated  by 
Delbceuf.  Again,  in  his  important  essay  on  TJie 
Influence  of  Isolation^  Weismann  concludes,  on  the 
basis  of  a  large  accumulation  of  facts,  that  the  con- 
stancy of  any  given  specific  type  '•  does  not  arise 
suddenly,  but  gradually,  and  is  established  by  the 
promiscuous  intercrossing  of  all  individuals."  From 
which,  he  says,  it  follows,  that  this  constancy  must 
cease  so  soon  as  the  condition  which  maintains  it 
ceases — i.  e.  so  soon  as  intercrossing  (Panmixia) 
between  all  individuals  ceases,  or  so  soon  as  a  portion 
of  a  species  is  isolated  from  its  parent  stock.  To 
this  principle  he  assigns  the  name  of  Amixia.  But 
Weismann's  Amixia  differs  from  my  Independent 
Variability  in  several  important  particulars;  and 
on  this  account  I  have  designedly  abstained  from 

1  The  passage  proceeds  to  show  that  in  view  of  this  consideration  we 
have  a  strong  additional  reason  for  rejecting  the  a  priori  do^mathat  all 
specific  characters  must  necessarily  be  useful  characters.  For  it  is  evident 
that  any  divergence  of  specific  character  which  is  brought  about  in  this 
way  need  not  present  any  utilitarian  significance— although,  of  course, 
natural  selection  will  ensure  that  it  shall  never  be  deleterious. 


Isolation.  13 

adopting  his  term.  Here  it  is  enough  to  remark 
that  it  answers  to  the  generic  term  Isolation,  with- 
out reference  to  the  kind  of  isolation  as  discriminate 
or  indiscriminate,  homogamous  or  apogamous.  On 
the  other  hand,  my  Independent  Variability  is  merely 
a  re-statement  of  the  so-called  "Law  of  Delbceuf," 
which,  in  his  own  words,  is  as  follows : —  . 

One  point,  however,  is  definitely  attained.  It  is  that  the 
proposition,  which  further  back  we  designated  paradoxical, 
is  rigorously  true.  A  constant  cause  of  variation,  however 
insignificant  it  may  be,  changes  the  uniformity  [of  type] 
little  by  little,  and  diversifies  it  ad  infinitum.  From  the  homo- 
geneous, left  to  itself,  only  the  homogeneous  can  proceed ;  but 
if  there  be  a  slight  disturbance  ["Idger  ferment"]  in  the 
homogeneous,  the  homogeneity  will  be  invaded  at  a  single 
point,  differentiation  will  penetrate  the  whole,  and,  after 
a  time— it  may  be  an  infinite  time— the  differentiation  will 
have  disintegrated  it  altogether. 

In  other  words,  the  "  Law,"  which  Delboeuf  has 
formulated  on  mathematical  grounds,  and  with  express 
reference  to  the  question  of  segregate  breeding, 
proves  that,  no  matter  how  infinitesimally  small  the 
difference  may  be  between  the  average  qualities  of 
an  isolated  section  of  a  species  compared  with  the 
average  qualities  of  the  rest  of  that  species,  if  the 
isolation  continues  sufficiently  long,  differentiation  of 
specific  type  is  necessarily  bound  to  ensue.  But,  to 
make  this  mathematical  law  biologically  complete,  it 
ought  to  be  added  that  the  time  required  for  the 
change  of  type  to  supervene  (supposing  apogamy  to 
be  the  only  agent  of  change)  will  be  governed  by  the 
range  of  individual  variability  which  the  species  in 
question  presents.  A  highly  stable  species  (such  as 
the  Goose)  might  require  an  immensely  long  time  for 


T4          Darwin,  and  after  Darwin. 

apogamy  alone  to  produce  any  change  of  type  in  an 
isolated  portion  of  the  species,  while  a  highly  variable 
species  (such  as  the  Ruff)  would  rapidly  change  in 
any  portion  that  might  be  indiscriminately  isolated. 
It  was  in  order  to  recognize  this  additional  and  very 
important  factor  that  I  chose  the  name  Independent 
Variability  whereby  to  designate  the  diversifying 
influence  of  merely  indiscriminate  isolation,  or  apo- 
gamy. Later  on  Mr.  Gulick  published  his  elaborate 
papers  upon  the  divergence  of  type  under  all  kinds  of 
isolation ;  and  retained  my  term  Independent,  but 
changed  Variability  into  Generation.  I  point  this 
out  merely  for  the  sake  of  remarking  that  his  In- 
dependent Generation  is  exactly  the  same  principle 
as  my  Independent  Variability,  and  Delbceuf 's  Mathe- 
matical Law. 

Now,  while  I  fully  agree  with  Mons.  Giard  where 
he  says,  in  the  introductory  lecture  of  his  course  on 
The  Factors  of  Evolution*,  that  sufficient  attention 
has  not  been  hitherto  given  by  naturalists  to  this 
important  factor  of  organic  evolution  (apogamy), 
I  think  I  have  shown  that  among  those  naturalists 
who  have  considered  it  there  is  a  sufficient  amount  of 
agreement.  Per  contra,  I  have  to  note  the  opinion 
of  Mr.  Wallace,  who  steadily  maintains  the  impossi- 
bility of  any  cause  other  than  natural  selection  (i.  e. 
one  of  the  forms  of  homogamy)  having  been  concerned 
in  the  evolution  of  species.  But  at  present  it  is  enough 
to  remark  that  even  Professor  Ray  Lankester — whose 
leanings  of  late  years  have  been  to  the  side  of  ultra- 
Darwinism,  and  who  is  therefore  disposed  to  agree 

1  Revue  ScitHtijiqtte,  Nov.  23,  1889. 


Isolation.  15 

with  Mr.  Wallace  wherever  this  is  logically  possible — 
even  Professor  Ray  Lankester  observes : — 

Mr.  Wallace  does  not,  in  my  judgement,  give  sufficient 
grounds  for  rejecting  the  proposition  which  he  indicates  as 
the  main  point  of  Mr.  Gulick's  valuable  essay  on  Divergent 
Evolution  through  Cumulative  Segregation.  Mr.  Gulick's 
idea  is  that  ....  no  two  portions  of  a  species  possess  exactly  the 
same  average  character,  and  the  initial  differences  will,  if  the 
individuals  of  the  two  groups  are  kept  from  intercrossing, 
assert  themselves  continuously  by  heredity  in  such  a  way  as 
to  ensure  an  increasing  divergence  of  the  forms  belonging  to 
the  two  groups,  amounting  to  what  is  recognized  as  specific 
distinction.  Mr.  Gulick's  idea  is  simply  the  recognition  of 
a  permanence  or  persistency  in  heredity,  which,  caeteris 
paribus,  gives  a  twist  or  direction  to  the  variations  of  the 
descendants  of  one  individual  as  compared  with  the  descendants 
of  another1. 

Now  we  have  seen  that  '•  Mr.  Gulick's  idea," 
although  independently  conceived  by  him,  had  been 
several  times  propounded  before ;  and  it  is  partly 
implicated  in  more  than  one  passage  of  the  Origin 
of  Species,  where  free  intercrossing,  or  the  absence  of 
isolation,  is  alluded  to  as  maintaining  the  constancy 
of  a  specific  type*.  Moreover,  it  is  still  more  fully 
recognized  in  the  last  edition  of  the  Variation  of 
Animals  and  Plants,  where  a  paragraph  is  added  for 
the  purpose  of  sanctioning  the  principle  in  the 
imperfect  form  that  it  was  stated  by  Weismann :(. 
Nevertheless,  to  Mr.  Gulick  belongs  the  credit  not 
only  of  having  been  the  first  to  conceive  (though  the 
last  to  publish)  the  "  idea  "  in  question,  and  of  having 
stated  it  with  greater  fullness  than  anybody  else  ;  but 

1  Nature,  Oct.  10,  1889,  p.  568.  «  e.  g.  p.  8l. 

1  See  Chapter  xxiii.  vol.  ii   p.  262.     (Edition  of  iSSS.) 


16          Darwin,  and  after  Darwin. 

still  more  of  having  verified  its  importance  as  a  factor 
of  organic  evolution. 

For,  in  point  of  fact,  Mr.  Gulick  was  led  to  his 
recognition  of  the  principle  in  question,  not  by  any 
deductive  reasoning  from  general  principles,  but  by 
his  own  particular  and  detailed  observations  of  the 
land  mollusca  of  the  Sandwich  Islands.  Here  there 
are  an  immense  number  of  varieties  belonging  to 
several  genera  ;  but  every  variety  is  restricted,  not 
merely  to  the  same  island,  but  actually  to  the  same 
valley.  Moreover,  on  tracing  this  fauna  from  valley 
to  valley,  it  is  apparent  that  a  slight  variation  in  the 
occupants  of  valley  a  as  compared  with  those  of  the 
adjacent  valley  i,  becomes  more  pronounced  in  the 
next — valley  3,  still  more  so  in  4,  &c.,  &c.  Thus  it 
was  possible,  as  Mr.  Gulick  says,  roughly  to  estimate 
the  amount  of  divergence  between  the  occupants  of 
any  two  given  valleys  by  measuring  the  number  of 
miles  between  them. 

As  already  stated,  I  have  myself  examined  his 
wonderful  collection  of  shells,  together  with  a  topo- 
graphical map  of  the  district ;  and  therefore  I  am  in 
a  position  to  testify  to  the  great  value  of  Mr.  Gulick's 
work  in  this  connexion,  as  in  that  of  the  utility 
question  previously  considered.  The  variations,  which 
affect  scores  of  species,  and  themselves  eventually  run 
into  fully  specific  distinctions,  are  all  more  or  less 
finely  graduated  as  they  pass  from  one  isolated 
region  to  the  next ;  and  they  have  reference  to 
changes  of  form  and  colour,  which  in  no  one  case 
presents  any  appearance  of  utility.  Therefore — and 
especially  in  view  of  the  fact  that,  as  far  as  he  could 
ascertain,  the  environment  in  the  different  valleys  was 


Isolation.  17 

essentially  the  same — no  one  who  examines  this 
collection  can  wonder  that  Mr.  Gulick  attributes  the 
results  which  he  has  observed  to  the  influence  of 
apogamy  alone,  without  any  reference  to  utility  or 
natural  selection. 

To  this  solid  array  of  remarkable  facts  Mr.  Wallace 
has  nothing  further  to  oppose  than  his  customary 
appeal  to  the  argument  from  ignorance,  grounded  on 
the  usual  assumption  that  no  principle  other  than 
natural  selection  can  be  responsible  for  even  the 
minutest  changes  of  form  or  colour.  For  my  own 
part,  I  must  confess  that  I  have  never  been  so  deeply 
impressed  by  the  dominating  influence  of  the  a  priori 
method  as  I  was  on  reading  Mr.  Wallace's  criticism 
of  Mr.  Gulick's  paper,  after  having  seen  the  material 
on  which  this  paper  is  founded.  To  argue  that  every 
one  of  some  twenty  contiguous  valleys  in  the  area  of 
the  same  small  island  must  necessarily  present  such 
differences  of  environment  that  all  the  shells  in  each 
are  differently  modified  thereby,  while  in  no  one  out 
of  the  hundreds  of  cases  of  modification  in  minute 
respects  of  form  and  colour  can  any  human  being 
suggest  an  adaptive  reason  therefor — to  argue  thus 
is  merely  to  affirm  an  intrinsically  improbable  dogma 
in  the  presence  of  a  great  and  consistent  array  of 
opposing  facts. 

I  have  laid  special  stress  on  this  particular  case  of 
the  Sandwich  Islands'  mollusca,  because  the  fifteen 
years  of  labour  which  Mr.  Gulick  has  devoted  to  their 
exhaustive  working  out  have  yielded  results  more 
complete  and  suggestive  than  any  which  so  far  have 
been  forthcoming  with  regard  to  the  effects  of  isolation 
in  divergent  evolution.  But,  if  space  permitted,  it 

III.  C 


i8          Darwin,  and  after  Darwin. 

would  be  easy  to  present  abundance  of  additional  facts 
from  other  sources,  all  bearing  to  the  same  conclusion 
—namely,  that  as  a  matter  of  direct  observation,  no 
less  than  of  general  reasoning,  any  unprejudiced  mind 
will  concede  to  the  principle  of  indiscriminate  isolation 
an  important  share  in  the  origination  of  organic  types. 
For  as  indiscriminate  isolation  is  thus  seen  sooner  or 
later  to  become  discriminate,  and  as  we  have  already 
seen  that  discriminate  isolation  is  a  necessary  condition 
to  all  or  any  modification,  we  can  only  conclude  that 
isolation  in  both  its  kinds  takes  rank  with  heredity 
and  variability  as  one  of  the  three  basal  principles 
of  organic  evolution. 

Having  got  thus  far  in  the  way  of  generalities,  we 
must  next  observe  sundry  further  matters  of  com- 
parative detail. 

i.  In  any  case  of  indiscriminate  isolation,  or 
apogamy,  the  larger  the  bulk  of  the  isolated  section 
the  more  nearly  must  its  average  qualities  resemble 
those  of  its  parent  stock ;  and,  therefore,  the  less 
divergence  of  character  will  ensue  in  a  given  time 
from  this  cause  alone.  For  instance,  if  one-fourth  of 
a  large  species  were  to  be  separated  from  the  other 
three-fourths  (say,  by  subsidence  causing  a  discon- 
tinuity of  area),  it  would  continue  the  specific  characters 
unchanged  for  an  indefinitely  long  time,  so  far  as 
the  influence  of  such  an  indiscriminate  isolation  is 
concerned.  But,  on  the  other  hand,  if  only  half  a 
dozen  individuals  were  to  be  thus  separated  from 
the  rest  of  their  species,  a  comparatively  short  time 
would  be  needed  for  their  descendants  to  undergo 
some  varietal  modification  at  the  hands  of  apogamy. 


Isolation.  19 

For,  in  this  case,  the  chances  would  be  infinitely 
against  the  average  characters  of  the  original  half- 
dozen  individuals  exactly  coinciding  with  those  of 
all  the  rest  of  their  species. 

2.  In   any   case   of   homogamy,   however,    it    is 
immaterial  what  proportional  number  of  individuals 
are  isolated  in  the  first  instance.     For  the  isolation  is 
here  discriminate,  or  effected  by  the  initial  difference 
of    the    average    qualities    themselves — a    difference, 
therefore,  which   presupposes   divergence   as   having 
already  commenced,  and   equally  bound  to  proceed 
whether  the  number   of  intergenerants   be   large  or 
small. 

It  may  here  be  remarked  that,  in  his  essay  on 
the  Influence  of  Isolation,  Professor  Weismann  fails 
to  distinguish  between  the  two  kinds  of  isolation. 
This  essay  deals  only  with  one  of  the  many  different 
forms  of  isolation — the  geographical — and  is  therefore 
throughout  concerned  with  a  consideration  of  diversity 
as  arising  from  apogamy  alone.  But  in  dealing  with 
this  side  of  the  matter  Weismann  anticipated  both 
Gulick  and  myself  in  pointing  out  the  law  of  inverse 
proportion,  which  I  have  stated  in  the  preceding 
paragraph  in  what  appears  to  me  its  strictly  accurate 
form. 

3.  Segregate  Breeding,  or  homogamy,  which  arises 
under  any  of  the  many  forms  of  discriminate  isolation, 
must  always  tend  to  be  cumulative.     For,  again  to 
quote  Mr.  Gulick,  who  has  constituted  this  fact  the 
most   prominent   as   it   is   the   most  original  feature 
of  his  essay,  "In  the  first  place,  every  new  form  of 
Segregation1  that  now  appears  depends  on,  and  is 

1  This  term  may  here  be  taken  as  equivalent  to  Isolation. 
C   3 


20          Darwin,  and  after  Darwin. 

superimposed  upon,  forms  of  Segregation  that  have 
been  previously  induced ;  for  when  Negative  Segre- 
gation arises  [i.e.  isolation  due  to  mutual  sterility], 
and  the  varieties  of  a  species  become  less  and  less 
fertile  with  one  another,  the  complete  infertility  that 
has  existed  between  them  and  some  other  species 
does  not  disappear,  nor  does  the  Positive  Segregation 
cease  [i.  e.  any  other  form  of  isolation  previously 
existing].  ...  In  the  second  place,  whenever 
Segregation  is  directly  produced  by  some  quality  of 
the  organism,  variations  that  possess  the  endowment 
in  a  superior  degree  will  have  a  larger  share  in  pro- 
ducing the  segregated  forms  of  the  next  generation, 
and  accordingly  the  segregative  endowment  of  the 
next  generation  will  be  greater  than  that  of  the 
present  generation ;  and  so  with  each  successive 
generation  the  segregation  will  become  increasingly 
complete."  And  to  this  it  may  be  added,  in  the 
third  place,  that  where  the  segregation  (isolation)  is 
due  to  the  external  conditions  of  life  under  which 
the  organism  is  placed,  or  where  it  is  due  to  natural 
selection  simultaneously  operating  in  divergent  lines 
of  evolution,  the  same  remarks  apply.  Hence  it 
follows  that  discriminate  isolation  is,  in  all  its  forms, 
cumulative. 

4.  The  next  point  to  be  noted  is,  that  the  cumu- 
lative divergence  of  type  thus  induced  can  take 
place  only  in  as  many  different  lines  as  there  are 
different  cases  of  isolation.  This  is  a  point  which 
Mr.  Gulick  has  not  expressly  noticed ;  but  it  is  one 
that  ought  to  be  clearly  recognized.  Seeing  that 
isolation  secures  the  breeding  of  similar  forms  by 
exclusion  (immediate  or  eventual)  of  those  which  are 


Isolation.  21 

dissimilar,  and  that  only  in  as  far  as  it  does  this 
can  it  be  a  factor  in  organic  evolution,  it  follows  that 
the  resulting  segregation,  even  though  cumulative, 
can  only  lead  to  divergence  of  organic  types  in  as 
many  directions  as  there  are  cases  of  isolation.  For 
any  one  group  of  intergenerants  only  serial  trans- 
formation is  possible,  even  though  the  transformation 
be  cumulative  through  successive  generations  in  the 
single  line  of  change.  But  there  is  always  a  probability 
that  during  the  course  of  such  serial  transformation 
in  time,  some  other  case  of  isolation  may  supervene, 
so  as  to  divide  the  previously  isolated  group  of  inter- 
generants into  two  or  more  further  isolated  groups. 
Then,  of  course,  opportunity  will  be  furnished  for 
divergent  transformation  in  space — and  this  in  as 
many  different  lines  as  there  are  now  different 
homogamous  groups. 

That  this  must  be  so  is  further  evident,  if  we 
reflect  that  the  evolutionary  power  of  isolation 
depends,  not  only  on  the  preventing  of  intercross- 
ing between  the  isolated  portion  of  a  species  and 
the  rest  of  that  species,  but  also  upon  the  permitting 
of  intercrossing  between  all  individuals  of  the  isolated 
portion,  whereby  the  peculiar  average  of  qualities  which 
they  as  a  whole  present  maybe  allowed  to  assert  itself 
in  their  progeny — or,  if  the  isolation  has  been  from  the 
first  discriminate,  whereby  the  resulting  homogamy 
may  thus  be  allowed  to  assert  itself.  Hence  any 
one  case  of  either  species  of  isolation,  discriminate  or 
indiscriminate,  can  only  give  rise  to  what  Mr.  Gulick 
has  aptly  called  "  monotypic  evolution,"  or  a  chain- 
like  series  of  types  arising  successively  in  time,  as 
distinguished  from  what  he  has  called  "polytypic 


22  Darwin,  and  after  Darwin. 

evolution,"  or  an  arborescent  multiplication  of  types 
arising  simultaneously  in  space. 

For  example,  let  us  again  take  the  geographical 
form  of  isolation.  Where  a  single  small  intergenerant 
group  of  individuals  is  separated  from  the  rest  of 
its  species— say,  on  an  oceanic  island — monotypic 
evolution  may  take  place  through  a  continuous  and 
cumulative  course  of  independent  variation  in  a 
single  line  of  change :  all  the  individuals  composing 
any  one  given  generation  will  closely  resemble  one 
another,  although  the  type  may  be  progressively 
altering  through  a  long  series  of  generations.  But  if 
the  original  species  had  had  two  small  colonies 
separated  from  itself  (one  on  each  of  two  different 
islands,  so  giving  rise  to  two  cases  of  isolation),  then 
polytypic  evolution  would  have  ensued  to  the  extent 
of  there  having  been  two  different  lines  of  evolu- 
tion going  on  simultaneously  (one  upon  each  of 
the  two  islands  concerned).  Similarly,  of  course,  if 
there  had  been  three  or  four  such  colonies,  there 
would  have  been  three  or  four  divergent  lines  of 
evolution,  and  so  on. 

5.  In  the  cases  of  isolation  just  supposed  there 
is  only  one  form  of  isolation  ;  and  it  is  thus  shown 
that  under  one  form  of  isolation  there  may  be  as 
many  lines  of  divergence  as  there  are  separate  cases 
of  such  isolation.  But  now  suppose  that  there  are 
two  or  more  forms  of  isolation — for  instance,  that 
on  the  same  oceanic  island  the  original  colony  has 
begun  to  segregate  into  secondary  groups  under 
the  influence  of  natural  selection,  sexual  selection, 
physiological  selection,  or  any  of  the  other  forms 
of  isolation — then  there  will  be  as  many  lines  of 


Isolation.  23 

divergent  evolution  going  on  at  the  same  time  (and 
here  on  the  same  area)  as  there  are  forms  of  isolation 
affecting  the  oceanic  colony.  And  this  because  each 
of  the  forms  of  isolation  has  given  rise  to  a  different 
case  of  isolation. 

Now,  inasmuch  as  different  forms  of  isolation,  when 
thus  superadded  one  to  another,  constitute  different 
cases  of  isolation,  we  may  lay  down  the  following 
general  law  as  applying  to  all  the  forms  of  isola- 
tion— namely,  The  number  of  possible  directions  in 
which  divergent  evolution  can  occur,  is  never  greater 
than,  though  it  may  be  equal  to,  the  number  of  cases 
of  efficient  isolation  —  or  tJie  number  of  efficiently 
separated  groups  of  intergenerants. 

6.  We  have  now  to  consider  with  some  care  the 
particular  and  highly  important  form  of  isolation 
that  is  presented  by  natural  selectioa  For  while 
this  form  of  isolation  resembles  all  the  other  forms 
of  the  discriminate  kind  in  that  it  secures  homo- 
gamy,  there  are  two  points  in  which  it  differs  from 
all  of  them,  and  one  point  in  which  it  differs  from 
most  of  them. 

Natural  selection  differs  from  all  the  other  known 
forms  of  isolation  (whether  discriminate  or  indis- 
criminate) in  that  it  has  exclusive  reference  to 
adaptations  on  the  one  hand,  and,  on  the  other  hand, 
necessitates  not  only  the  elimination,  but  the  de- 
struction of  the  excluded  individuals.  Again,  natural 
selection  differs  from  most  of  the  other  forms  of 
isolation  in  that,  unless  assisted  by  some  other 
form,  it  can  never  lead  to  polytypic,  but  only 
to  monotypic  evolution.  The  first  two  points  of 
difference  are  here  immaterial ;  but  the  last  is  one 


24          Darwin,  and  after  Darwin. 

of  the  highest  importance,  as  we  shall  immediately 
perceive. 

In  nearly  all  the  other  forms  of  isolation,  polytypic 
or  divergent  evolution  may  arise  under  the  influence 
of  that  form  alone,  or  without  the  necessary  co- 
operation of  any  other  form.  This  we  have  already 
seen,  for  example,  in  regard  to  geographical  isolation, 
under  which  there  may  be  as  many  different  lines 
of  transmutation  going  on  simultaneously  as  there 
are  different  cases  of  isolation — say,  in  so  many 
different  oceanic  islands.  Again,  in  regard  to  physio- 
logical isolation  the  same  remark  obviously  applies ; 
for  it  is  evident  that  even  upon  the  same  geographical 
area  there  may  be  as  many  different  lines  of  trans- 
mutation going  on  simultaneously  as  there  are  cases 
of  this  form  of  isolation.  The  bar  of  mutual  sterility, 
whenever  and  wherever  it  occurs,  must  always  render 
polytypic  evolution  possible.  And  so  it  is  with  almost 
all  the  other  forms  of  isolation:  that  is  to  say,  one 
form  does  not  necessarily  require  the  assistance  of 
another  form  in  order  to  create  an  additional  case 
of  isolation.  But  it  is  a  peculiarity  of  natural  selec- 
tion, considered  as  a  form  of  isolation,  that  it  does 
necessarily  require  the  assistance  of  some  other  form 
before  it  can  give  rise  to  an  additional  case  of  isola- 
tion; and  therefore  before  it  can  give  rise  to  any 
divergence  of  character  in  ramifying  lines,  as  distin- 
guished from  transformation  of  characters  in  a  single 
line.  Or,  in  other  words,  natural  selection,  when  act- 
ing alone,  can  never  induce  polytypic  evolution,  but 
only  monotypic. 

That  this  important  conclusion  is  a  necessary 
deduction  from  the  theory  of  natural  selection  itself, 


Isolation.  25 

a  very  few  words  will  be  enough  to  show.  For, 
according  to  the  theory,  survival  of  the  fittest  is 
a  form  of  isolation  which  acts  through  utility,  by 
destroying  all  the  individuals  whom  it  fails  to  isolate. 
Hence  it  follows  that  survival  of  the  fittest  is  a  form 
of  isolation  which,  if  acting  alone,  cannot  possibly 
effect  divergent  evolution.  For,  in  the  first  place, 
there  is  nothing  in  this  form  of  isolation  to  ensure 
that  the  fitter  individuals  should  fail  to  interbreed 
with  the  less  fit  which  are  able  to  survive;  and,  in 
the  second  place,  in  all  cases  where  the  less  fit  are 
not  sufficiently  fit  to  be  suffered  to  breed,  they  are 
exterminated— i.  e.  not  permitted  to  form  a  distinct 
variety  of  their  own.  If  it  be  said  that  survival  of 
the  fittest  may  develop  simultaneously  two  or  more 
lines  of  useful  change,  the  answer  is  that  it  can 
only  do  this  if  each  of  the  developing  varieties  is 
isolated  from  the  others  by  some  additional  form 
of  isolation ;  for,  if  not,  there  can  be  no  commence- 
ment of  utilitarian  divergence,  since  whatever  number 
of  utilitarian  changes  may  be  in  course  of  simul- 
taneous development,  they  must  in  this  case  be  all 
blended  together  in  a  single  line  of  specific  trans- 
mutation. Nay,  even  if  specific  divergence  has 
actually  been  commenced  by  natural  selection  when 
associated  with  some  other  form  of  homogamy,  if 
the  latter  should  afterwards  be  withdrawn,  natural 
selection  would  then  be  unable  to  maintain  even  so 
much  divergence  of  character  as  may  already  have 
been  attained  :  free  intercrossing  between  the  two 
collateral,  and  no  longer  isolated  branches,  would 
ensure  their  eventual  blending  into  a  common  stock. 
Therefore,  I  repeat,  natural  selection,  when  acting 


26          Danvin,  and  after  Darwin. 

alone,  can  never  induce  polytypic  evolution,  but 
only  monotypic. 

Now  I  regret  to  say  that  here,  for  the  first  and 
only  time  throughout  the  whole  course  of  the 
present  treatise,  I  find  myself  in  seeming  opposition 
to  the  views  of  Darwin.  For  it  was  the  decidedly 
expressed  opinion  of  Darwin  that  natural  selection 
is  competent  to  effect  polytypic,  or  divergent,  evo- 
lution. Nevertheless,  I  believe  that  the  opposition 
is  to  a  large  extent  only  apparent,  or  due  merely  to 
the  fact  that  Darwin  did  not  explicitly  state  certain 
considerations  which  throughout  his  discussion  on 
"divergence  of  character"  are  seemingly  implied. 
But,  be  this  as  it  may.  I  have  not  even  appeared 
to  desert  his  leadership  on  a  matter  of  such  high  im- 
portance without  having  duly  considered  the  question 
in  all  its  bearings,  and  to  the  utmost  limit  of  my 
ability.  Moreover,  about  two  years  after  the  publica- 
tion of  my  first  paper1  upon  the  subject,  Mr.  Gulick 
followed,  at  somewhat  greater  length,  in  the  same  line 
of  dissent.  Like  all  the  rest  of  his  work,  this  is  so 
severely  logical  in  statement,  as  well  as  profoundly 
thought  out  in  substance,  that  I  do  not  see  how  it 
is  possible  for  any  one  to  read  impartially  what  he 
has  written,  and  then  continue  to  hold  that  natural 
selection,  if  unassisted  by  any  other  form  of  isola- 
tion, can  possibly  effect  divergence  of  character — 
or  polytypic  as  distinguished  from  monotypic  evo- 
lution 2. 

I  may  here  quote  from  Mr.  Gulick's  paper  three 
propositions,  serving  to  state  three  large  and  general 

1  Ztol.  Journal  Lin.  Soc.,  vol.  xix.  pp.  337-411. 

2  Ilnd.,  Vol.  XX.  pp.  202-213. 


Isolation.  27 

bodies  of  observable  fact,  which  severally  and  collec- 
tively go  to  verify,  with  an  overwhelming  mass  of 
evidence,  the  conclusion  previously  reached  on  grounds 
of  general  reasoning. 

The  facts  of  geographical  distribution  seem  to  me  to  justify 
the  following  statements  : — 

(1)  A  species  exposed  to  different  conditions  in  the  different 
parts  of  the  area  over  which  it  is  distributed,  is  not  repre- 
sented   by  divergent    forms   when    free    interbreeding    exists 
between  the  inhabitants  of  the  different  districts.     In  other 
words,  Diversity  of  Natural  Selection  without  Separation  does 
not  produce  divergent  evolution. 

(2)  We  find  many  cases  in  which   areas,  corresponding  in 
the   character  of  the  environment,  but  separated  from  each 
other  by  important  barriers,  are  the  homes  of  divergent  forms 
of  the  same  or  allied  species. 

(3)  In  cases  where  the  separation  has  been  long  continued, 
and  the  external  conditions  are   the   most  diverse   in   points 
that  involve  diversity  of  adaptation,  there  we  find  the  most 
decided  divergences  in  the  organic  forms.     That  is,   where 
Separation    and    Divergent    Selection    have  long  acted,   the 
results  are  found  to  be  the  greatest. 

The  ist  and  3rd  of  these  propositions  will  probably  be 
disputed  by  few,  if  by  any.  The  proof  of  the  2nd  is 
found  wherever  a  set  of  closely  allied  organisms  is  so 
distributed  over  a  territory  that  each  species  and  variety 
occupies  its  own  narrow  district,  within  which  it  is  shut  by 
barriers  that  restrain  its  distribution,  while  each  species  of 
the  environing  types  is  distributed  over  the  whole  territory. 
The  distribution  of  terrestrial  molluscs  on  the  Sandwich 
Islands  presents  a  great  body  of  facts  of  this  kind. 


CHAPTER   II. 

ISOLATION  (continited). 

I  WILL  now  recapitulate  the  main  doctrines  which 
have  been  set  forth  in  the  foregoing  chapter,  and  then 
proceed  to  consider  the  objections  which  have  been 
advanced  against  them. 

It  must  be  remembered  that  by  isolation  I  mean 
exactly  what  Mr.  Gulick  does  by  "  Segregation," 
and  approximately  what  Professor  Weismann  does 
by  "  Amixia  " — i.  e.  the  prevention  of  intercrossing. 

Isolation  occurs  in  very  many  forms  besides  the 
geographical,  as  will  be  more  fully  shown  at  the  end 
of  this  chapter ;  and  in  all  its  forms  it  admits  of 
degrees. 

It  also  occurs  in  two  very  different  species  or 
kinds — namely,  discriminate  and  indiscriminate.  These 
I  have  called  respectively  Homogamy  and  Apogamy. 
This  all-important  distinction  has  been  clearly  recog- 
nized by  Mr.  Gulick,  as  a  result  of  his  own  thought 
and  observation,  independently  of  anything  that  I  have 
published  upon  the  subject. 

In  view  of  this  distinction  Isolation  takes  rank  with 
Heredity  and  Variability  as  one  of  the  most  funda- 
mental principles  of  organic  evolution.  For,  if  these 


Isolation.  29 

other  two  principles  be  granted,  the  whole  theory 
of  descent  resolves  itself  into  an  inquiry  touching  the 
causes,  forms,  and  degrees  of  Homogamy. 

Save  in  cases  where  very  large  populations  are 
concerned,  apogamy  must  sooner  or  later  give  rise 
per  se  to  homogamy,  owing  to  the  Law  of  Delboeuf, 
which  is  the  principle  that  I  have  called  Indepen- 
dent Variability,  and  Gulick  has  called  Independent 
Generation.  But  of  course  this  does  not  hinder  that 
under  apogamy  various  other  causes  of  homogamy 
are  likely  to  arise — in  particular  natural  selection. 

That  natural  selection  differs  from  most  of  the  other 
forms  of  isolation  in  not  being  capable  of  causing 
divergent  or  polytypic  evolution  must  at  once  become 
evident,  if  we  remember  that  the  only  way  in  which 
isolation  of  any  form  can  cause  such  evolution  is  by 
partitioning  a  given  group  of  intergenerants  into  two 
or  more  groups,  each  of  which  is  able  to  survive  as 
thus  separated  from  the  other,  and  so  to  carry  on  the 
evolution  in  divergent  lines.  But  the  distinguishing 
peculiarity  of  natural  selection,  considered  as  a  form 
of  isolation,  is  that  it  effects  the  isolation  by  killing 
off  all  the  individuals  which  it  fails  to  isolate :  con- 
sequently, this  form  of  isolation  differs  from  other 
forms  in  prohibiting  the  possibility  of  any  ramification 
of  a  single  group  of  intergenerants  into  two  or  more 
groups,  for  the  purpose  of  carrying  on  the  evolution 
in  divergent  lines.  Therefore,  under  this  form  of 
isolation  alone,  evolution  must  proceed,  palm-like,  in 
a  single  line  of  growth.  So  to  speak,  the  successive 
generations  continuously  ascend  to  higher  things  on 
the  steps  supplied  by  their  own  "  dead  selves  "  ;  but 
in  doing  so  they  must  climb  a  single  ladder,  no 


30          Darwin,  and  after  Darwin. 

rung  of  which  can  be  allowed  to  bifurcate  in  the 
presence  of  the  uniformity  secured  for  that  generation 
by  the  free  intercrossing  of  the  most  fit.  Even 
though  beneficial  variations  may  arise  in  two  or  more 
directions  simultaneously,  and  all  be  simultaneously 
selected  by  survival  of  the  fittest,  the  effect  of  free 
intercrossing  (in  the  absence  of  any  other  form  of 
isolation)  will  be  to  fuse  all  these  beneficial  variations 
into  one  common  type,  and  so  to  end  in  monotypic 
evolution  as  before.  In  order  to  secure  polytypic 
evolution,  intercrossing  between  the  different  bene- 
ficial variants  which  may  arise  must  be  prevented  ; 
and  there  is  nothing  to  prevent  such  intercrossing  in 
the  process  of  natural  selection  per  se.  In  order  that 
the  original  group  of  intergenerants  should  be  divided 
and  sub-divided  into  two  or  more  groups  of  inter- 
generants, some  additional  form  of  isolation  must 
necessarily  supervene  —  when,  of  course,  polytypic 
evolution  will  result.  And,  as  Mr.  Gulick  has  shown, 
the  conclusion  thus  established  by  deductive  reason- 
ing is  verified  inductively  by  the  facts  of  geographical 
distribution. 

How,  then,  are  we  to  account  for  the  fact  that 
Darwin  attributed  to  natural  selection  the  power  to 
cause  divergence  of  character?  The  answer  is  suffi- 
ciently simple.  He  does  so  by  tacitly  invoking  the  aid 
of  some  other  form  of  homogamy  in  every  case.  If  we 
careiully  read  pp.  86-97  °f  tne  Origin  of  Species,  where 
this  subject  is  under  consideration,  we  shall  find  that 
in  every  one  of  the  arguments  and  illustrations  which 
are  adduced  to  prove  the  power  cf  natural  selection  to 
efiect !C  divergence  of  character,"  he  either  pre-supposes 
or  actually  names  some  other  form  of  homogamy  us 


Isolation.  31 

the  originating  cause  of  the  diversity  that  is  afterwards 
presented  to  natural  selection  for  further  intensification. 
To  give  only  one  example.  At  the  starting-point  of 
the  whole  discussion  the  priority  of  such  other  forms 
of  homogamy  is  assumed  in  the  following  words  : — 

But  how,  it  may  be  asked,  can  any  analogous  principle 
[to  that  of  diversity  caused  by  artificial  selection]  apply  in 
nature  ?  I  believe  it  can  and  does  apply  most  efficiently 
(though  it  was  a  long  time  before  I  saw  how),  from  the 
simple  circumstance  that  the  more  diversified  the  descendants 
from  any  one  species  become  in  structure,  constitution,  and 
habits,  by  so  much  will  they  be  better  enabled  to  seize  on 
many  and  widely  diversified  places  in  the  polity  of  nature, 
and  so  be  enabled  to  increase  in  numbers. 

Now,  without  question,  so  soon  as  segregate 
breeding  in  two  or  more  lines  of  homogamy  has  been 
in  any  sufficient  degree  determined  by  some  "  change 
of  structure,  constitution,  or  habits."'  natural  selection 
will  forthwith  proceed  to  increase  the  divergence  in 
as  many  different  lines  as  there  are  thus  yielded  dis- 
criminately  isolated  sections  of  the  species.  And  this 
fact  it  must  have  been  that  Darwin  really  had  before 
his  mind  when  he  argued  that  diversification  of  char- 
acter is  caused  by  natural  selection,  through  the  benefit 
gained  by  the  diversified  forms  being  thus  fi  enabled 
to  increase  in  number."  Nevertheless  he  does  not  ex- 
pressly state  the  essential  point,  that  although  diversi- 
fication of  character,  when  once  begun>  is  thus  promoted 
by  natural  selection,  which  forthwith  proceeds  to  cul- 
tivate each  of  the  resulting  branches,  yet  diversifica- 
tion of  character  can  never  be  originated  by  natural 
selection.  The  change  of  "structure,"  of  "constitution," 
of  "  habits,"  of  "  station,"  of  geographical  area,  of  reci- 


32          Darwin,  and  after  Darwin. 

procal  fertility,  and  so  on — this  change,  whatever  it 
may  have  been,  must  clearly  have  been  antecedent  to 
any  operation  of  natural  selection  through  the  benefit 
which  arose  from  the  change.  Therefore  the  change 
must  in  all  cases  have  been  due,  in  the  first  instance, 
to  some  other  form  of  isolation  than  the  superadded 
form  which  afterwards  arose  from  superior  fitness 
in  the  possession  of  superior  benefit — although,  so 
long  as  the  prior  form  of  isolation  endured,  or  con- 
tinued to  furnish  the  necessary  condition  to  the  co- 
operation of  survival  of  the  fittest,  survival  of  the 
fittest  would  have  continued  to  increase  the  divergence 
of  character  in  as  many  ramifying  lines  as  there  were 
thus  given  to  its  action  separate  cases  of  isolation 
by  other  means. 

In  short,  as  divergence  of  character  must  in  all  cases 
be  due  to  a  prevention  of  intercrossing,  and  as  in  the 
process  of  natural  selection  there  is,  ex  hypothesi, 
nothing  to  prevent  the  intercrossing  until  the  diver- 
gence has  already  arisen,  to  suppose  that  natural 
selection  alone  can  have  caused  the  divergence,  is  to 
suppose  that  natural  selection  can  have  caused  the 
conditions  of  its  own  activity,  which  is  absurd. 

Seeing,  then,  that  even  in  cases  where  any  "  benefit " 
arises  from  divergence  of  character,  such  benefit  can 
arise  only  after  the  divergence  has  already  commenced, 
and  seeing  that  on  this  as  on  other  accounts  previously 
mentioned  it  is  plainly  impossible  to  attribute  the 
origin  of  such  divergence  to  natural  selection,  we  find 
that  natural  selection  must  be  in  all  cases  assisted 
by  some  other  form  of  isolation,  if  it  is  to  be  con- 
cerned in  polytypic  as  distinguished  from  monotypic 
evolution.  But  this  does  not  hinder  that,  when  it 


Isolation.  33 

is  so  assisted,  natural  selection  may  become— and, 
I  believe,  does  become — the  most  efficient  of  all 
the  forms  of  isolation  in  promoting  divergence  of 
character.  For,  in  the  first  place,  of  all  the  forms 
of  isolation  natural  selection  is  probably  the  most 
energetic  in  promoting  monotypic  evolution;  so  that 
under  the  influence  of  such  isolation  monotypic 
evolution  probably  advances  more  rapidly  than 
it  does  under  any  other  form  of  isolation.  In  the 
second  place,  when  polytypic  evolution  has  been 
begun  by  any  of  these  other  forms  of  isolation,  and 
natural  selection  then  sets  to  work  on  each  of  the 
resulting  branches,  although  natural  selection  is  thus 
engaged  in  as  many  different  acts  of  monotypic  evolu- 
tion as  there  are  thus  separate  cases  supplied  to  it  by 
these  other  forms  of  isolation,  the  joint  result  of  all 
these  different  acts  is  to  hurry  on  the  polytypic 
evolution  which  was  originally  started  by  the  other 
forms  of  isolation.  So  to  speak,  natural  selection  is 
the  forcing  heat,  acting  simultaneously  on  each  of  the 
separate  branches  which  has  been  induced  to  sprout 
by  other  means ;  and  in  thus  rapidly  advancing  the 
growth  of  all  the  branches,  it  is  still  entitled  to  be 
regarded  as  the  most  important  single  cause  of  diver- 
sification in  organic  nature,  although  we  must  hence- 
forth cease  to  regard  it  as  in  any  instance  the 
originating  cause — or  even  so  much  as  the  sustaining 
cause. 

So  much  by  way  of  summary  and  recapitulation. 
I  will  now  briefly  consider  the  only  objections 
which,  so  far  as  I  can  see,  admit  of  being  brought 
against  the  foregoing  doctrine  of  Isolation  as  held 
by  Mr.  Gulick  and  myself.  These  possible  objections 

III  P 


34          Darwin,  and  after  Darwin. 

are  but  two  in  number — although  but  one  of  them 
has  been  hitherto  adduced.  This,  therefore,  I  will 
take  first. 

Mr.  Wallace,  with  his  customary  desire  to  show 
that  natural  selection  is  everywhere  of  itself  capable 
of  causing  organic  evolution,  seeks  to  minimize  the 
swamping  effects  of  free  intercrossing,  and  the  conse- 
quent importance  of  other  forms  of  isolation.  His 
argument  is  as  follows. 

Alluding  to  the  researches  of  Mr.  J.  A.  Allen, 
and  others,  on  the  amount  of  variation  presented 
by  individuals  of  a  species  in  a  state  of  nature, 
Mr.  Wallace  shows  that,  as  regards  any  given  part  of 
the  animal  under  consideration,  there  is  always  to 
be  found  a  considerable  range  of  individual  variation 
round  the  average  mean  which  goes  to  constitute  the 
specific  character  of  the  type.  Thus,  for  example, 
Mr.  Allen  says  of  American  birds,  "that  a  varia- 
tion of  from  fifteen  to  twenty  per  cent,  in  general  size, 
and  an  equal  degree  of  variation  in  the  relative  size 
of  different  parts,  may  be  ordinarily  expected  among 
specimens  from  the  same  species  and  sex,  taken  at 
the  same  locality,  while  in  some  cases  the  variation 
is  even  greater  than  this."  Now,  Mr.  Wallace  is  under 
the  impression  that  these  facts  obviate  the  difficulty 
which  arises  from  the  presence  of  free  intercrossing — 
the  difficulty,  that  is.  against  the  theory  of  natural 
selection  when  natural  selection  is  supposed  to  have 
been  the  exclusive  means  of  modification.  For,  as 
he  says,  "  if  less  size  of  body  would  be  beneficial, 
then,  as  half  the  variations  in  size  are  above  and 
half  below  the  mean  or  existing  standard  of  the 
species,  there  would  be  ample  beneficial  variations "  ; 


Isolation.  35 

and  similarly  with  regard  to  longer  or  shorter  legs, 
wings,  tails,  &c.,  darker  or  lighter  colour,  and  so  on 
through  all  the  parts  of  any  given  organism. 

Well,  although  I  have  no  wish  at  all  to  disparage 
the  biological  value  of  these  actual  measurements 
of  the  range  of  individual  variation,  I  must  point 
out  that  they  are  without  any  value  at  all  in  the 
connexion  which  Mr.  Wallace  adduces  them.  We 
did  not  require  these  measurements  to  tell  us  the 
broad  and  patent  fact  that  "  no  being  on  this  earthly 
ball  is  like  another  all  in  all" — or,  in  less  Tenny- 
sonian  words,  that  as  regards  every  specific  structure 
there  is  a  certain  amount  of  individual  variability 
round  an  average  mean.  Indeed,  in  my  own  paper 
on  Physiological  Selection — against  which  Mr.  Wal- 
lace is  here  specially  arguing—  I  expressly  said,  as 
previously  remarked,  "that  a  specific  type  may 
be  regarded  as  the  average  mean  of  all  individual 
variations"  The  fact  of  such  individual  variability 
round  a  specific  mean  has  always  been  well  known 
to  anatomists  ;  it  constitutes  one  of  the  basal  pillars 
of  the  whole  Darwinian  theory ;  and  is  besides  a 
matter  of  universal  recognition  as  regards  human 
stature,  features,  and  so  forth.  The  value  of  Mr. 
Allen's  work  consists  in  accurately  measuring  the 
amount  or  range  of  individual  variation ;  but  the 
question  of  its  amount  or  range  is  without  relevancy 
in  the  present  connexion.  For  the  desirability  of 
isolation  as  an  aid  to  natural  selection  even  where 
monotypic  evolution  is  concerned,  does  not  arise 
with  any  reference  to  the  amount  or  range  of  variation : 
it  arises  with  reference  to  the  number  of  variations 
which  are — or  are  not — similar  and  simultaneous.  If 


36          Darwin,  and  after  Darwin. 

there  be  a  sufficient  number  which  are  both  similar 
and  simultaneous,  the  desirability  of  any  co-operating 
form  of  isolation  is  correspondingly  removed,  because 
natural  selection  may  then  have  sufficient  material 
wherewith  to  overcome  the  adverse  influence  of  free 
intercrossing,  and  so  of  itself  to  produce  monotypic 
evolution.  Now,  variations  may  be  numerous,  similar, 
and  simultaneous,  either  on  account  of  some  common 
cause  acting  on  many  individuals  at  the  same  time, 
or  on  account  of  the  structures  in  question  being 
more  or  less  variable  round  a  specific  mean.  In 
the  latter  case — which  is  the  only  case  that  Mr. 
Allen's  measurements  have  to  do  with— the  law  of 
averages  will  of  course  determine  that  half  the  whole 
number  of  variations  in  any  given  structure,  in  any 
given  generation,  will  be  above  the  mean  line.  But, 
equally  of  course,  no  one  has  ever  denied  that  where, 
for  either  of  these  reasons,  natural  selection  is  pro- 
vided with  sufficient  material,  it  is  correspondingly 
capable  of  improving  the  specific  type  without 
the  assistance  of  any  other  form  of  homogamy ; 
so  to  speak,  they  protect  themselves  by  their  very 
numbers,  and  their  superiority  over  others  leads  to 
their  survival  and  accumulation.  But  what  is  the 
result  ?  The  result  can  only  be  monotypic  evolution, 
No  matter  how  great  the  number,  or  how  great  the 
range,  of  variations  round  an  average  specific  mean, 
out  of  such  material  natural  selection  can  never 
produce  polytypic  evolution  :  it  may  change  the  type 
to  any  extent  during  successive  generations,  and 
in  a  single  line  of  change  ;  but  it  cannot  branch 
the  type,  unless  some  other  form  of  homogamy 
intervenes.  Therefore,  when  Mr.  Wallace  adduces 


Isolation.  37 

the  well-known  fact  that  all  structures  vary  more 
or  less  round  a  specific  mean  as  proof  that  natural 
selection  need  not  be  incommoded  by  free  inter- 
crossing, but  can  of  itself  produce  all  the  known 
phenomena  of  specific  evolution,  he  fails  to  perceive 
that  his  argument  refers  only  to  one  aspect  of  such 
evolution  (viz.  the  transformation  of  species  in  time), 
and  does  not  apply  to  the  aspect  with  which  alone 
my  paper  on  Physiological  Selection  was  concerned 
(viz.  the  multiplication  of  species  in  space). 

The  same  thing  may  be  shown  in  this  way.  It  is 
perfectly  obvious  that  where  the  improvement  of  type 
in  a  linear  series  is  concerned  (monotypic  evolution), 
free  intercrossing,  far  from  being  a  hindrance  to  the 
process,  is  the  very  means  by  which  the  process  is 
accomplished.  Improvement  here  ascends  by  suc- 
cessive steps,  in  successive  generations,  simply  because 
of  the  general  intercrossing  of  the  generally  most  fit. 
with  the  result  that  the  species,  as  a  whole,  gradually 
becomes  transformed  into  another  species,  as  a  whole. 
Therefore,  it  would  be  mere  fatuity  in  any  one  to 
adduce  free  intercrossing  as  a  ''  difficulty "  against 
natural  selection  alone  being  competent  to  produce 
evolution  of  this  kind.  But  where  the  kind  of 
evolution  is  that  whereby  the  species  is  differentiated 
— where  it  is  required,  for  instance,  to  produce  different 
structures  in  different  portions  of  the  species,  such  as 
the  commencement  of  a  fighting  spur  on  the  wing  of 
a  duck,  or  novel  characters  of  any  sort  in  different 
groups  of  the  species— free  intercrossing  is  no  longer 
a  condition  to,  but  an  absolute  preventive  of,  the 
process ;  and,  therefore,  unless  checked  as  between 
each  portion  of  the  species  by  some  form  of  homo- 

f  34-23 


38  Darwin,  and  after  Darwin. 

gamy  other  than  natural  selection,  it  must  effectually 
inhibit  any  segregation  of  specific  types,  or  divergence 
of  character. 

Hence  it  is  that,  while  no  Darwinian  has  ever 
questioned  the  power  of  unaided  selection  to  cause 
improvement  of  character  in  successive  generations,  in 
common  now  with  not  a  few  other  Darwinians  I  have 
emphatically  denied  so  much  as  the  abstract  possi- 
bility of  selection  alone  causing  a  divergence  of  char- 
acter in  two  or  more  simultaneous  lines  of  change. 

And,  although  these  opposite  views  cannot  be 
reconciled,  I  am  under  the  impression  that  they  do 
admit  of  being  explained.  For  I  take  them  to 
indicate  a  continued  failure  to  perceive  the  all-im- 
portant distinction  between  evolution  as  monotypic 
and  polytypic.  Unless  one  has  fully  grasped  this 
distinction,  and  constantly  holds  it  in  mind,  he  is 
not  in  a  position  to  understand  the  "difficulty"  in 
question ;  nor  can  he  avoid  playing  fast  and  loose 
with  natural  selection  as  possibly  the  sole  cause  of 
evolution,  and  as  necessarily  requiring  the  co-operation 
of  some  other  cause.  But  if  he  once  clearly  perceives 
that  "evolution"  is  a  logical  genus,  of  which  the  mono- 
typic and  the  polytypic  forms  are  species,  he  will 
immediately  escape  from  his  confusion,  and  find  that 
while  the  monotypic  form  may  be  caused  by  natural 
selection  alone  the  polytypic  form  can  never  be 
so  caused. 

The  second  difficulty  which  I  have  to  mention  as  at 
first  sight  attaching  to  the  views  of  Mr.  Gulick  and 
myself  on  the  subject  of  Isolation  is,  that  in  an  iso- 
lated section  of  a  species  Mr.  Francis  Galton's  law  of 


Isolation.  39 

regression  in  the  average  character  of  offspring  to 
the  typical  character  of  the  group  through  reversion 
or  atavism  (Natural  Inheritance,  p.  97)  must  have 
the  effect  of  neutralizing  the  segregative  influence  of 
mere  apogamy.  That  such,  however,  cannot  be  the 
case  has  been  well  shown  by  Mr.  Gulick  in  his  paper 
on  Intensive  Segregation.  Without  at  all  disputing 
the  validity  of  Mr.  Galton's  law,  he  proves  that "  it  can 
hold  in  full  force  only  where  there  is  free  crossing, 
otherwise  no  divergent  race  could  ever  be  formed  by 
any  amount  of  selection  and  independent  breeding1." 
This  is  so  self-evident  that  I  need  not  quote  his  demon- 
stration of  the  point. 

In  conclusion,  then,  and  having  regard  to  the 
principle  of  isolation  as  a  whole,  or  in  all  the  many  and 
varied  forms  in  which  this  principle  obtains,  I  trust  that 
I  have  redeemed  the  promise  with  which  I  set  out — 
viz.  to  show  that  in  relation  to  the  theory  of  descent 
this  principle  is  of  an  importance  second  to  no  other, 
not  even  excepting  heredity,  variability,  and  the 
struggle  for  existence.  This  has  now  been  fully 
shown,  inasmuch  as  we  have  clearly  seen  that  the  im- 
portance of  the  struggle  for  existence,  and  consequent 
survival  of  the  fittest,  arises  just  because  survival 
of  the  fittest  is  a  form,  and  a  very  stringent  form,  of 
isolation  ;  while,  as  regards  both  heredity  and  vari- 
ability, we  are  now  in  a  position  to  see  that  the  more 
fully  we  recognize  their  supreme  importance  as 
principles  concerned  in  organic  evolution,  the  more 
must  we  also  recognize  that  any  rational  theory  of 
such  evolution  becomes,  in  the  last  resort,  a  theory 

1  Zool.  Journal  Lin.  Soc.,  vol.  xxiii.  p.  313. 


40  Darwin,  and  after  Darwin. 

of  the  different  modes  in  which  efficient  isolation  can 
be  secured.  For,  in  whatever  degree  the  process  of 
organic  evolution  has  been  dependent  upon  heredity 
with  variability,  in  that  degree  must  it  also  have  been 
dependent  upon  the  means  of  securing  homogamy, 
whereby  alone  the  force  of  heredity  can  be  made  to 
expend  itself  in  the  innumerable  directions  of  pro- 
gressive change,  instead  of  continually  neutralizing  the 
force  of  variability  by  promiscuous  intercrossing. 


CHAPTER   III. 

PHYSIOLOGICAL  SELECTION. 

So  far  we  have  been  concerned  with  the  principle 
of  Isolation  in  general.  We  have  now  to  consider 
that  form  of  isolation  which  arises  in  consequence  of 
mutual  infertility  between  the  members  of  any  group 
of  organisms  and  those  of  all  other  similarly  isolated 
groups  occupying  simultaneously  the  same  area. 

Against  the  view  that  natural  selection  is  a  sufficient 
explanation  of  the  origin  of  species,  there  are  two 
fatal  difficulties:  one,  the  contrast  between  natural 
species  and  domesticated  varieties  in  respect  of  cross- 
sterility  ;  the  other,  the  fact  that  natural  selection 
cannot  possibly  give  rise  to  polytypic  as  distinguished 
from  monotypic  evolution.  Now  it  is  my  belief  that 
the  theory  of  physiological  selection  fully  meets  both 
these  difficulties.  Indeed  I  hold  this  to  be  undeniable 
in  a  formal  or  logical  sense :  the  only  question  is  as 
to  the  evidence  which  can  be  adduced  for  the  theory 
in  a  practical  or  biological  sense.  Therefore  in  this 
chapter,  where  the  theory  has  first  of  all  to  be  stated, 
I  shall  restrict  the  exposition  as  much  as  possible 
to  the  former,  leaving  for  subsequent  consideration  the 
biological  side. 


42          Darwin,  and  after  Darwin 

The  following  is  a  brief  outline  sketch  of  this 
theory l. 

Of  all  parts  of  those  variable  objects  which  we 
call  organisms,  the  most  variable  is  the  reproductive 
system  ;  and  the  variations  may  carry  with  them  func- 
tional changes,  which  may  be  either  in  the  direction 
of  increased  or  of  diminished  fertility.  Consequently 
variations  in  the  way  of  greater  or  less  fertility  fre- 
quently take  place,  both  in  plants  and  animals ;  and 
probably,  if  we  had  adequate  means  of  observing  this 
point,  we  should  find  that  there  is  no  one  variation 
more  common.  But  of  course  where  infertility  arises 
— whether  as  a  result  of  changed  conditions  of  life,  or, 
as  we  say,  spontaneously — it  immediately  becomes 
extinguished,  seeing  that  the  individuals  which  it 
affects  are  less  able  (if  able  at  all)  to  propagate  and 
to  hand  on  the  variation.  If,  however,  the  variant, 
while  showing  some  degree  of  infertility  with  the  parent 
form,  continues  to  be  as  fertile  as  before  when  mated 
with  similar  variants,  under  these  circumstances  there 
is  no  reason  why  such  differential  fertility  should  not 
be  perpetuated. 

Stated  in  another  form  this  suggestion  enables  us 
to  regard  many,  if  not  most,  species  as  the  records  of 
variationsin  the  reproductive  systems  of  their  ancestors. 
When  variations  of  a  non-useful  kind  occur  in  any 
of  the  other  systems  or  parts  of  organisms,  they  are, 
as  a  rule,  immediately  extinguished  by  intercrossing. 
But  whenever  they  arise  in  the  reproductive  system 
in  the  way  here  suggested,  they  tend  to  be  preserved 
as  new  natural  varieties,  or  incipient  species.  At 
first  the  difference  would  only  be  in  respect  of  the 

1  See  Nineteenth  Century,  January,  1887,  pp.  61,  6a. 


Physiological  Selection.  43 

reproductive  systems  ;  but  eventually,  on  account  of 
independent  variation,  other  differences  would  super- 
vene, and  the  variety  would  take  rank  as  a  true 
species. 

Now  we  must  remember  that  physiological  isola- 
tion is  not  like  those  other  forms  of  isolation  (e.  g 
geographical)  which  depend  for  their  occurrence  on 
accidents  of  the  environment,  and  which  may  therefore 
take  place  suddenly  in  a  full  degree  of  complete- 
ness throughout  a  large  section  of  a  species.  Physio- 
logical isolation  depends  upon  distinctive  characters 
belonging  to  organisms  themselves;  and  it  would 
be  opposed  to  the  whole  theory  of  descent  with 
progressive  modification  to  imagine  that  absolute 
sterility  usually  arises  in  a  single  generation  between 
two  sections  of  a  perfectly  fertile  species.  Therefore 
evolutionists  must  believe  that  in  most,  if  not  in  all 
cases — could  we  trace  the  history,  say  of  any  two 
species,  which  having  sprung  from  a  single  parent 
stock  on  a  common  area,  are  now  absolutely  sterile 
with  one  another  we  should  find  that  this  mutual 
sterility  had  been  itself  a  product  of  gradual  evolution. 
Starting  from  complete  fertility  within  the  limits  of  a 
single  parent  species,  the  infertility  between  derivative 
or  divergent  species,  at  whatever  stage  in  their  evolution 
this  began  to  occur,  must  usually  at  first  have  been  well- 
nigh  imperceptible,  and  thenceforth  have  proceeded 
to  increase  stage  by  stage. 

But,  if  it  be  true  that  physiological  isolation  between 
genetically  allied  groups  must  usually  itself  have  been 
the  product  of  a  gradual  evolution  ;  and  if,  when 
fully  evolved,  it  constitutes  a  condition  of  the  first 
importance  to  any  further  differentiation  of  these 


44          Darwin,  and  after  Darwin. 

groups  (by  preventing  fusion  again  into  one  group, 
more  or  less  resembling  the  original  parent  form),  do 
we  not  perceive  at  least  a  strong  probability  that 
in  the  lower  stages  of  its  evolution  such  mutual  in- 
fertility must  have  acted  as  a  segregating  influence 
between  the  diverging  types,  in  a  degree  proportional 
to  its  own  development  ?  The  importance  of  mutual 
sterility  as  a  condition  to  divergent  evolution  is  not 
denied,  when  this  sterility  is  already  present  in  an 
absolute  degree ;  and  we  have  just  seen  that,  before 
it  can  have  attained  to  this  absolute  degree,  //  must 
presumably \  and  as  a  rule,  itself  have  been  the  subject 
of  a  gradual  development.  Does  it  not  therefore 
become,  on  merely  antecedent  grounds,  in  a  high 
degree  probable,  that  from  the  moment  of  its  in- 
ception this  isolating  agency  must  have  played  the 
part  of  a  segregating  cause,  in  a  degree  propor- 
tional to  that  of  its  completeness  as  a  physiological 
character  ? 

Whoever  answers  this  question  in  the  affirmative 
will  have  gone  most  of  the  way  towards  accepting,  on 
merely  antecedent  grounds,  the  theory  of  physiological 
selection.  And  therefore  it  is  that  I  have  begun  this 
statement  of  the  theory  by  introducing  it  upon  these 
grounds,  thereby  hoping  to  show  how  extremely  simple 
— how  almost  self-evident — is  the  theory  which  it  will 
now  be  my  endeavour  to  substantiate.  I  may  here 
add  that  the  theory  was  foreshadowed  by  Mr.  Belt 
in  1 874  l,  clearly  enunciated  in  its  main  features  by 
Mr.  Catchpool  in  i8842,  and  very  fully  thought  out 
by  Mr.  Gulick  during  a  period  of  about  fifteen  years, 

1  Nicaragua,  p.  207. 
1  Nature,  voL  xxxi.  p.  4. 


Physiological  Selection.  45 

although  he  did  not  publish  until  a  year  after  the 
appearance  of  my  own  paper  in  1 886 1. 

I  must  next  proceed  to  state  some  of  the  leading 
features  of  physiological  selection  in  further  detail. 

It  has  already  been  shown  that  Darwin  clearly 
perceived  that  the  very  general  occurrence  of  some 
degree  of  infertility  between  allied  species  cannot 
possibly  be  attributed  to  the  direct  agency  of  natural 
selection.  His  explanation  was  that  the  slight  struc- 
tural modifications  entailed  by  the  transformation  ol 
one  specific  type  into  another,  so  react  upon  the 
highly  delicate  reproductive  system  of  the  changing 
type  as  to  render  it  in  some  degree  infertile  with 
its  parent  type.  Now  the  theory  of  physiological 
selection  begins  by  traversing  this  view.  It  does 
not,  however,  deny  that  in  some  cases  the  morpho- 
logical may  be  the  prior  change ;  but  it  strenuously 
denies  that  this  must  be  so  in  all  cases.  Indeed, 
according  to  my  statement  in  1886,  the  theory  inclines 
to  the  view  that  as  a  rule,  the  physiological  change 
is  prior.  At  the  same  time,  the  theory,  as  I  have 
always  stated  it,  maintains  that  it  is  immaterial  whether, 
"in  the  majority  of  instances/'  the  physiological  change 
has  been  prior  to  the  morphological,  or  vice  versa ; 
since  in  either  case  the  physiological  change  will 
equally  make  for  divergence  of  character. 

1  Zool.  Journal,  Lin.  Soc.}  vol.  xix.  pp.  337-411  ^1886);  and  for 
Mr.  Guliok's  papers,  ibid.,  vol.  xx.  pp.  189-274  (1887),  vol.  xxiii. 
pp.  312-380  (1889).  Mr.  Gulick  has  recently  drawn  my  attention,  in 
a  private  letter,  to  the  fact  that  as  early  as  1872  a  paper  of  his  was 
read  at  the  British  Association,  bearing  the  title  Diversity  of  Evolution 
under  one  set  of  External  Conditions,  and  that  here  the  principle  of 
physiological  segregation  is  stated.  Although  it  does  not  appear  that 
Mr.  Gulick  then  appreciated  the  great  importance  of  this  principle,  it 
entitles  him  to  claim  priority. 


46          Darwin,  and  after  Darwin. 

To  show  this  clearly  the  best  way  will  be  to  consider 
the  two  cases  separately,  taking  first  that  in  which 
the  physiological  change  has  priority.  In  this  case 
our  theory  regards  any  morphological  changes  which 
afterwards  supervene  as  due  to  the  independent  varia- 
bility which  will  sooner  or  later  arise  under  the 
physiological  isolation  thus  secured.  But  to  what- 
ever causes  the  subsequent  morphological  changes 
may  be  due,  the  point  to  notice  is  that  they  are  as 
a  general  rule,  consequent  upon  the  physiological 
change.  For  in  whatever  degree  such  infertility  arises 
between  two  sections  of  a  species  occupying  the  same 
area,  in  that  degree  is  their  interbreeding  prevented, 
and,  therefore,  opportunity  is  given  for  a  subsequent 
divergence  of  type,  whether  by  the  influence  of  inde- 
pendent variability  alone,  or  also  by  that  of  natural 
selection,  as  now  acting  more  or  less  independently 
on  each  of  the  partially  separated  groups.  In  short, 
all  that  was  said  in  the  foregoing  chapters  with  respect 
to  isolation  in  general,  here  applies  to  physiological 
isolation  in  particular ;  and  by  supposing  such  isola- 
tion to  have  been  the  prior  change,  we  can  as  well  un- 
derstand the  subsequent  appearance  of  morphological 
divergence  on  continuous  areas,  as  in  other  forms 
of  isolation  we  can  understand  such  divergence  on 
discontinuous  areas,  seeing  that  even  a  moderate 
degree  of  cross-infertility  may  be  as  effectual  for 
purposes  of  isolation  as  a  high  mountain-chain,  or 
a  thousand  miles  of  ocean. 

Here,  then,  are  two  sharply-defined  theories  to 
explain  the  very  general  fact  of  there  being  some 
greater  or  less  degree  of  cross-infertility  between  allied 
species.  The  older,  and  hitherto  current  theory, 


Physiological  Selection.  47 

supposes  the  cross-infertility  to  be  but  an  accident 
of  specific  divergence,  which,  therefore,  has  nothing 
to  do  with  causing  the  divergence.  The  newer  theory, 
on  the  other  hand,  supposes  the  cross-infertility  to 
have  often  been  a  necessary  condition  to  the  diverg- 
ence having  begun  at  all.  Let  us  now  consider  which 
theory  has  most  evidence  in  its  favour. 

First  of  all  we  have  to  notice  the  very  general 
occurrence  of  the  fact  in  question.  For  when  we 
include  the  infertility  of  hybrids,  as  well  as  first 
crosses,  the  occurrence  of  some  degree  of  infertility 
between  allied  species  is  so  usual  that  Mr.  Wallace 
recommends  experiments  to  ascertain  whether  careful 
observation  might  not  prove,  even  of  species  which 
hybridize,  <;  that  such  species,  when  crossed  with  their 
near  allies,  do  always  produce  offspring  which  are 
more  or  less  sterile  inter  se  V  This  seems  going  too 
far,  but  nevertheless  it  is  the  testimony  of  a  highly 
competent  naturalist  to  the  very  general  occurrence  of 
an  association  between  the  morphological  differentia- 
tion of  species  and  the  fact  of  a  physiological  isolation. 
Now  I  regard  it  as  little  short  of  self-evident  that  this 
general  association  between  mutual  infertility  and 
innumerable  secondary,  or  relatively  variable  mor- 
phological distinctions,  is  due  to  the  former  having 
been  an  original  and  a  necessary  condition  to  the 
occurrence  of  the  latter,  in  cases  where  intercrossing 
has  not  been  otherwise  prevented. 

The  importance  of  physiological  isolation,  when 
once  fjilly  developed,  cannot  be  denied,  for  it  is  evident 
that  if  such  isolation  could  be  suddenly  destroyed 
between  two  allied  species  occupying  a  common  area, 

1  Darwinism,  p.  169. 


48  Darwin,  and  after  Darwin. 

they  would  sooner  or  later  become  fused  into  a 
common  type — supposing,  of  course,  no  other  form 
of  isolation  to  be  present.  The  necessity  then  for 
this  physiological  form  of  isolation  in  maintaining 
a  specific  differentiation  which  has  been  already  at- 
tained cannot  be  disputed.  Yet  it  has  been  regarded 
as  "  Darwinian  heresy "  to  suggest  that  it  can  have 
been  of  any  important  service  during  the  process  of 
attainment,  or  while  the  specific  differentiation  is 
being  advanced,  and  this  notwithstanding  that  the 
physiological  change  must  presumably  have  developed 
pari passu  with  the  morphological,  and  notwithstand- 
ing that  in  countless  cases  the  former  is  associated 
with  every  conceivable  variety  of  the  latter. 

Again,  why  should  the  physiological  change  be 
thus  associated  with  every  conceivable  variety  of 
morphological  change  ?  Throughout  the  length  and 
breadth  of  both  vegetable  and  animal  kingdoms  we 
find  this  association,  in  the  great  majority  of  cases, 
where  new  species  arise.  Therefore,  on  the  supposi- 
tion that  in  all  such  cases  the  physiological  change 
has  been  adventitiously  induced  by  the  morpho- 
logical changes,  we  have  to  face  an  apparently  unan- 
swerable question — Why  should  the  reproductive 
mechanism  of  all  organic  beings  have  been  thus 
arranged,  as  it  were,  to  change  in  immediate  response 
to  the  very  slightest  alteration  in  the  complex  har- 
mony of  "  somatic  "  processes,  which  now  more  than 
ever  is  recognized  as  exercising  so  comparatively 
little  influence  on  the  hereditary  endowments  of  this 
mechanism  ?  Consider  the  difference  between  a  worm 
and  the  bird  that  is  eating  it,  an  oak  tree  and  the 
gall-insect  that  is  piercing  it :  are  we  to  suppose  that 


Physiological  Selection.  49 

in  all  cases,  no  matter  how  greatly  the  types  differ, 
they  must  agree  in  this,  that  when  any  parts  of 
these  complex  structures  change,  ever  so  slightly, 
the  reproductive  system  is  almost  certain  to  be 
adventitiously  affected,  yet  always  thus  affected  in 
the  same  peculiar  way  ? 

If  it  be  answered  that  the  reproductive  system  is 
known  to  be  very  sensitive  to  slight  changes  in  the 
external  conditions  of  life,  the  answer  proves  too 
much.  For  though  this  is  true,  yet  our  opponents 
must  acknowledge  that  the  reproductive  system  is 
not  so  sensitive,  in  this  particular  respect,  as  their 
interpretation  of  the  origin  of  specific  infertility 
requires.  The  proof  of  this  point  is  overwhelming, 
for  there  is  the  evidence  from  the  entire  range  of  our 
domesticated  productions,  both  vegetable  and  animal. 
Here  the  amount  of  structural  change,  which  has  been 
slowly  accumulated  by  artificial  selection,  is  often 
much  greater  in  amount,  and  incomparably  more 
rapid,  than  that  which  has  been  induced  between 
allied  species  by  natural  selection  ;  and  yet  there  is 
scarcely  any  indication  of  the  reproductive  system 
having  been  affected  in  the  particular  way  that  our 
opponents'  theory  requires.  There  are  many  in- 
stances of  its  having  been  affected  in  sundry  other 
ways  (chiefly,  however,  without  any  accompanying 
morphological  change)  ;  but  among  all  the  thousands 
of  our  more  or  less  enormously  modified  artificial 
types,  there  is  scarcely  one  instance  of  such  a  peculiar 
sexual  relation  between  the  modified  descendants  of 
a  common  type  as  so  usually  obtains  between  allied 
species  in  nature.  Yet  in  all  other  respects  evolu- 
tionists are  bound  to  believe  that  the  process  of 

III.  E 


50          Darwin,  and  after  Darwin. 

modification  has  been  in  both  cases  strictly  analogous 
Why  then  this  conspicuous  difference  with  respect  to 
the  reproductive  system  ? 

The  answer  is  simple.  It  has  never  been  the  object 
of  breeders  or  of  horticulturists  to  select  variations 
in  the  direction  of  cross-infertility,  for  the  swamping 
effects  of  intercrossing  are  much  more  easily  and 
rapidly  prevented  by  artificial  isolation.  Consequently, 
although  they  have  been  able  to  modify  natural  types 
in  so  many  directions  and  in  such  high  degrees  with 
regard  to  morphology ',  there  has  been  no  accompanying 
physiological  modification  of  the  kind  required.  But 
in  nature  there  is  no  such  thing  as  artificial,  i.e.  in- 
tentional, isolation.  Consequently,  on  common  areas 
it  must  usually  happen  that  those  changes  of  mor- 
phology which  are  associated  with  cross-infertility 
are  the  only  ones  which  can  arise.  Hence  the  very 
remarkable  contrast  between  our  domesticated  varie- 
ties and  natural  species  with  regard  to  cross-infertility 
is  just  what  the  present  theory  would  expect,  or, 
indeed,  require.  But  on  any  other  theory  it  has 
hitherto  remained  inexplicable. 

In  particular,  the  contrast  in  question  has  consti- 
tuted one  of  the  main  difficulties  with  which  the  theory 
of  natural  selection  has  hitherto  had  to  contend,  not 
only  in  the  popular  mind,  but  also  in  the  judgement 
of  naturalists,  including  the  joint-authors  of  the  theory 
themselves.  Thus  Darwin  says  : — 

The  fertility  of  varieties  is,  with  reference  to  my  theory, 
of  equal  importance  with  the  sterility  of  species,  for  it  seems 
to  make  a  broad  and  clear  distinction  between  varieties  and 
species  *. 

1  Origin  of  Species,  p.  236. 


Physiological  Selection.  51 

And  Mr.  Wallace  says:— 

One  of  the  greatest,  or  perhaps  we  may  say  the  greatest,  of 
all  the  difficulties  in  the  way  of  accepting  the  theory  of  natural 
selection  as  a  complete  explanation  of  the  origin  of  species,  has 
been  the  remarkable  difference  between  varieties  and  species  in 
respect  of  fertility  when  crossed 1. 

Now,  in  view  of  this  conspicuous  contrast,  Darwin 
suggested  that  species  in  .a  state  of  nature  "  will  have 
been  exposed  during  long  periods  of  time  to  more 
uniform  conditions  than  have  domesticated  varieties, 
and  [that]  this  may  well  make  a  wide  difference  in  the 
result."  Now  we  have  to  remember  that  species,  living 
and  extinct,  are  numbered  by  millions,  and  represent 
every  variety  of  type,  constitution,  and  habits ;  is 
it  probable,  then,  that  this  one  peculiarity  of  the 
reproductive  system  should  be  due,  in  so  many  cases, 
to  some  merely  incidental  effect  produced  on  that 
system  by  uniform  conditions  of  life?  Again,  ex 
hypothesi,  at  the  time  when  a  variety  is  first  forming, 
the  influence  exercised  by  uniform  conditions  of  life 
(whatever  in  different  cases  this  may  happen  to  be) 
cannot  be  present  as  regards  that  variety  :  yet  this  is 
just  the  time  when  its  infertility  with  the  parent  (or 
allied)  form  is  most  likely  to  have  arisen  ;  for  it  is 
just  then  that  the  nascent  variety  would  otherwise 
have  been  most  liable  to  extinction  by  free  inter- 
crossing— even  supposing  that  in  the  presence  of  such 
intercrossing  the  variety  could  ever  have  come  into 
existence  at  all. 

Mr.  Wallace  meets  the  difficulty  by  arguing  that 
sterility  between  allied  species  may  have  been  brought 
about  by  the  direct  influence  of  natural  selection. 

1  Darwinism,  p.  152. 
£   2 


52          Darwin,  and  after  Darwin. 

But,  as  previously  remarked,  this  view  is  expressly 
opposed  to  that  of  Darwin,  who  held  that  Wallace's 
contention  is  erroneous. 

It  will  be  seen,  then,  that  both  Darwin,  and  Wallace, 
fully  recognize  the  necessity  of  finding  some  explana- 
tion of  the  infertility  of  allied  species,  over  and  above 
the  mere  reaction  of  morphological  differentiation  on 
the  physiology  of  the  reproductive  system,  and  they 
both  agree  in  suggesting  additional  causes,  though 
they  entirely  disagree  as  to  what  these  causes  are. 
Now,  the  theory  of  physiological  selection  likewise 
suggests  an  additional  cause — or,  rather,  a  new  ex- 
planation— and  one  which  is  surely  the  most  probable. 
For  what  is  to  be  explained?  The  very  general 
association  of  a  certain  physiological  peculiarity  with 
that  amount  of  morphological  change  which  dis- 
tinguishes species  from  species,  of  whatever  kind  the 
change  may  be,  and  in  whatever  family  of  the  animal 
or  vegetable  kingdom  it  may  occur.  Well,  the  theory 
of  physiological  selection  explains  this  very  general 
association  by  the  simple  supposition  that,  at  least 
in  a  large  number  of  cases,  it  was  the  physiological 
peculiarity  which  first  of  all  led  to  the  morphological 
divergence,  by  interposing  the  bar  of  sterility  between 
two  sections  of  a  previously  uniform  species ;  and  by 
thus  isolating  the  two  sections  one  from  another, 
started  each  upon  a  subsequently  independent  course 
of  divergent  evolution. 

Or,  to  put  it  in  another  way,  if  the  occurrence  of 
this  physiological  peculiarity  has  been  often  the  only 
possible  means  of  isolating  two  sections  of  a  species 
occupying  a  common  area,  and  thus  giving  rise  to 
a  divergence  of  specific  type  (as  obviously  must  have 


Physiological  Selection.  53 

been  the  case  wherever  there  was  an  absence  of  any 
other  form  of  isolation),  it  is  nothing  less  than  a 
necessary  consequence  that  many  allied  species  should 
now  present  the  physiological  peculiarity  in  question. 
Thus  the  association  between  the  physiological  pecu- 
liarity and  the  morphological  divergence  is  explained 
by  the  simple  hypothesis,  that  the  former  has  acted 
as  a  necessary  condition  to  the  occurrence  of  the 
latter.  In  the  absence  of  other  forms  of  isolation, 
the  morphological  divergence  could  not  have  taken 
place  at  all,  had  not  the  physiological  peculiarity 
arisen ;  and  hence  it  is  that  we  now  meet  with  so 
many  cases  where  such  divergence  is  associated  with 
this  peculiarity. 

So  far  we  have  been  considering  the  physiological 
change  as  historically  the  prior  one.  Here,  at  first 
sight,  it  may  seem  that  the  segregative  power  of 
physiological  selection  must  end ;  for  it  may  well 
seem  impossible  that  the  physiological  change  can 
ever  be  necessary  for  the  divergence  of  morphological 
varieties  into  true  species  in  cases  where  it  has  not 
been  the  prior  change,  but  has  only  set  in  after  mor- 
phological changes  have  proceeded  far  enough  to  have 
already  constituted  definite  varieties.  A  little  thought, 
however,  will  show  that  physiological  selection  is  quite 
as  potent  a  condition  to  the  differentiation  of  species 
when  it  occurs  after  varietal  divergence  has  begun,  as 
it  is  when  it  occurs  before  the  divergence — and  hence 
that  it  really  makes  no  difference  to  the  theory  of 
physiological  selection  whether,  in  particular  cases,  the 
cross-infertility  arises  before  or  after  any  structural  or 
other  modifications  with  which  it  is  associated. 


54          Darwin,  and  after  Darwin. 

For  the  theory  does  not  assert  that  all  varieties 
have  been  due  to  physiological  selection.  There  are 
doubtless  many  other  causes  of  the  origin  of  varieties 
besides  cross-infertility  with  parent  forms;  but,  as 
a  general  rule,  it  does  not  appear  that  they  are  by 
themselves  capable  of  carrying  divergence  beyond 
a  merely  varietal  stage.  In  order  to  carry  divergence 
to  the  stage  of  producing  species,,  it  appears  to  be 
a  general  condition  that,  sooner  or  later,  cross-infertility 
should  arise — seeing  that,  when  varieties  do  succeed 
in  becoming  species,  we  almost  invariably  find  that, 
as  a  matter  of  fact,  cross-infertility  has  arisen.  Hence, 
if  cross-infertility  has  thus  usually  been  a  necessary 
condition  to  a  varietal  divergence  becoming  specific, 
it  can  make  no  material  difference  when  the  incipient 
infertility  arose. 

It  may  be  asked,  however,  whether  I  suppose  that, 
when  the  physiological  change  is  subsequent,  it  is 
directly  caused  by  change  of  structure,  size,  colour,  &c., 
or  that  it  arises,  so  to  speak,  accidentally,  from  other 
causes  which  may  have  affected  the  sexual  system  in 
the  required  way.  To  this  question  I  may  briefly 
reply,  that,  looking  to  the  absence  of  any  influence 
exercised  on  the  reproductive  systems  of  our  domesti- 
cated plants  and  animals  by  the  great  and  varied 
changes  which  so  many  of  these  forms  present,  it 
would  seem  that  among  natural  varieties  such  closely 
analogous  changes  are  presumably  not  the  usual  causes 
of  the  physiological  change,  even  where  the  latter  are 
subsequent  to  the  former.  Nevertheless.  I  do  not 
deny  that  in  some  of  these  cases  changes  of  structure, 
size,  colour,  &c.,  may  be  the  causes  of  the  physiological 
change  by  reacting  on  the  sexual  system  in  the  re- 


Physiological  Selection.  55 

quired  way.  But  in  such  cases  free  intercrossing  will 
have  prevented  the  perpetuation  of  any  morphological 
changes,  save  those  which  have  the  power  of  so  re- 
acting on  the  reproductive  system  as  to  produce  the 
physiological  change,  and  thus  to  protect  themselves 
against  the  full  and  adverse  power  of  free  intercrossing. 
We  know  that  slight  or  initial  changes  of  structure, 
colour,  &c.,  frequently  occur  as  varieties,  and  yet  that 
on  common  areas  very  few  of  these  varieties  become 
distinct  species :  free  intercrossing  prevents  any  such 
further  divergence  of  character.  But  if  in  the  course 
of  many  such  abortive  attempts,  as  it  were,  to  produce 
a  new  species,  nature  happens  to  hit  upon  a  structural 
or  a  colour  variation  which  is  capable  of  reacting  on 
the  sexual  system  in  the  particular  way  required,  then 
this  variation  will  be  enabled  to  protect  itself  against 
free  intercrossing  in  proportion  to  its  own  development. 
Or,  in  other  words,  the  more  it  develops  as  a  morpho- 
logical change,  the  more  will  it  increase  the  physio- 
logical change ;  while  the  more  the  physiological 
change  is  thus  increased,  the  more  will  it  in  turn 
promote  the  morphological.  By  such  action  and 
reaction  the  development  of  each  furthers  the  develop- 
ment of  the  other,  till  from  an  almost  imperceptible 
variety,  apparently  quite  fertile  with  its  parent  form, 
there  arises  a  distinct  species  absolutely  sterile  with 
its  parent  form.  In  such  cases,  therefore,  it  is  still 
the  physiological  conditions  which  have  selected  the 
particular  morphological  changes  capable  of  so  react- 
ing on  the  reproductive  system  as  to  produce  cross- 
infertility,  and  thus  to  protect  themselves  against  the 
destructive  power  of  free  intercrossing.  So  to  speak, 
free  intercrossing  is  always  on  the  watch  to  level 


56          Darwin,  and  after  Darwin. 

down  any  changes  which  natural  selection,  or  any 
other  cause  of  varietal  divergence,  may  attempt  to 
produce;  and  therefore,  in  order  to  produce — or  to 
increase — such  divergence  in  the  absence  of  any  other 
form  of  isolation,  natural  selection  must  hit  upon  such 
changes  of  structure,  form,  or  colour,  as  are  so  cor- 
related with  the  reproductive  system  as  to  create  the 
physiological  isolation  that  is  required. 

To  show  how  the  principle  of  selective  fertility 
may  be  combined  with  what  apparently  is  the  most 
improbable  form  of  isolation  for  this  purpose — the 
geographical — I  quote  the  following  suggestion  made 
by  Professor  Lloyd  Morgan  in  his  Animal  Life  and 
Intelligence . — 

Suppose  two  divergent  local  varieties  were  to  arise  in 
adjacent  areas,  and  were  subsequently  (by  stress  of  competition 
or  by  geographical  changes)  driven  together  into  a  single 
area.  ...  If  their  unions  be  fertile,  the  isolation  will  be  an- 
nulled by  intercrossing — the  two  varieties  will  form  one  mean  or 
average  variety.  But  if  the  unions  be  infertile,  the  isolation 
will  be  preserved,  and  the  two  varieties  will  continue  separate. 
Suppose  now,  and  the  supposition  is  by  no  means  an  improb- 
able one,  that  this  has  taken  place  again  and  again  in  the 
evolution  of  species ;  then  it  is  clear  that  those  varietal  forms 
which  had  continued  to  be  fertile  together  would  he  swamped 
by  intercrossing  ;  while  those  varietal  forms  which  had  become 
infertile  would  remain  isolated.  Hence,  in  the  long  run,  iso- 
lated forms  occupying  a  common  area  would  be  infertile. 
(P-  107.) 

If  then  cross-sterility  may  thus  arise  even  in  associ- 
ation with  geographical  isolation,  may  it  not  also 
arise  in  its  absence  ?  And  may  it  not  thus  give  rise 
to  the  differentiation  of  varieties  on  account  of  this 
physiological  isolation  alone? 


Physiological  Selection.  57 

Only  two  further  points  need  be  mentioned  to 
make  this  statement  of  physiological  selection  as 
complete  as  the  present  rtsumt  of  its  main  principles 
requires. 

The  first  is,  that,  as  Mr.  Wallace  remarks,  "  every 
species  has  come  into  existence  coincident  both  in 
space  and  time  with  a  pre-existing  and  closely  allied 
species."  I  regard  this  as  important  evidence  that 
physiological  selection  is  one  of  the  natural  causes 
concerned.  For  the  general  fact  implied  is  that  every 
species  has  come  into  existence  on  an  area  occupied 
by  its  parent  type,  and  therefore  under  circumstances 
which  render  it  imperative  that  intercrossing  with  that 
type  should  be  prevented.  In  the  case  of  monotypic 
evolution  by  natural  selection  alone,  intercrossing 
with  the  parent  type  is  prevented  through  the  gradual 
extinction  of  that  type  by  successive  generations  of 
the  developing  type.  But  in  the  case  of  polytypic 
evolution,  intercrossing  with  the  parent  type  can 
only  be  prevented  by  some  form  of  isolation  other 
than  natural  selection  ;  and  here  it  is  evident  that 
cross-infertility  with  the  parent  type  must  be  as 
efficient  to  that  end  as  any  other  form  of  isolation 
that  can  be  imagined.  Consequently  we  might 
almost  have  expected  beforehand .  that  in  a  large 
proportional  number  of  cases  cross-infertility  should 
have  been  the  means  employed.  And  the  fact  that 
this  is  actually  the  case  so  far  corroborates  the  only 
theory  which  is  able  to  explain  it. 

The  second  point  is  this. 

It  appears  to  be  comparatively  rare  for  any  cause 
of  specific  divergence  to  prove  effectual  on  common 
areas,  unless  it  sooner  or  later  becomes  associated  with 


58  Danvin,  and  after  Darwin. 

some  degree  of  cross-infertility.  But  through  this 
association,  the  segregating  influence  of  both  the 
causes  concerned  is,  as  Mr.  Gulick  has  shown,  greatly 
increased.  For  instance,  if  the  segregating  influence 
of  some  degree  of  cross-infertility  be  associated  with 
that  of  any  other  form  of  isolation,  then,  not  only 
will  the  two  segregating  influences  be  added,  but 
multiplied  together.  And  thus,  by  their  mutual 
action  and  reaction,  divergent  evolution  is  promoted 
at  a  rapidly  increasing  rate. 

I  will  now  summarize  the  main  points  of  the  theory 
of  physiological  isolation  in  a  categorical  form. 

1.  If  no  other  form  of  isolation  be  present,  specific 
divergence  can  only  take  place  when  some  degree  of 
cross-infertility  has  previously  arisen  between  two  or 
more  sections  of  a  species. 

2.  When  such   cross-infertility   has   arisen  it  may 
cause  specific  divergence,  either  (a)  by  allowing  in- 
dependent variability  in  each  of  the  physiologically 
isolated  groups  ;  (b]  by  becoming  associated  with  any 
other  cause  of  differentiation  already  operating ;    or 
(c]  by  both  these  means  combined. 

3.  As   some   degree   of   cross-infertility  generally 
obtains  between   allied   species,   we   are  justified    in 
concluding  that    this    has    been   the    most    frequent 
— or,  at  any  rate,  the  most  effective— kind  of  isola- 
tion where  the  origin  of  species  is  concerned ;   and 
therefore    the    kind    with    which,    in    the    case    of 
species-formation,    natural    selection,    or    any    other 
cause  of  specific  divergence,  has  been   most  usually 
associated. 

4.  Where    varietal   divergence  has  begun   in   the 


Physiological  Selection.  59 

absence  of  cross-infertility,  such  divergence  seems,  as 
a  general  rule,  to  have  been  incapable  of  attaining  to  a 
specific  value. 

5.  Therefore,  in  the  vast  majority  of  such  cases,  it 
must  have  been  those  varietal  changes  of  structure, 
size,  colour,  &c.,  which  happened  to  have  afterwards 
been  assisted  by  the  reproductive  change  that  were 
on  this  account  selected  as  successful  candidates  for 
specific  differentiation. 

6.  It  follows,  that  it  makes  no   difference   to   the 
general  theory  of  physiological  selection  in  what  pro- 
portion of  cases  the  physiological  change  has  been 
the  initial  change  ;   for,  whether  prior  or  subsequent 
to  the  varietal  changes  with  which  it  becomes  associ- 
ated, its  presence  has   been  equally  important  as  a 
condition  to  specific  divergence. 

7.  When  physiological  isolation  becomes  associated 
with  natural  selection,  or  any  other  form  of  homogamy, 
the  segregative  power  of  both  is  augmented.     More- 
over, so   great   is  the   augmentation   that  even  very 
moderate  degrees  of  physiological  isolation— them- 
selves capable  of  effecting  little  or  nothing — become 
very  powerful  when  associated  with  moderate  degrees 
of  any  other  kind  of  homogamy,  and  vice  versa. 

8.  The    theory    of    physiological    selection    effec- 
tually  explains   the   divergent   evolution   of  specific 
types   and  the  cross-infertility  of  such   types   when 
evolved. 

To  prevent,  if  possible,  the  continuance  of  certain 
misunderstandings  with  regard  to  my  original  state- 
ment of  the  new  theory,  let  me  here  disclaim  some 
views  which  have  been  assigned  to  me.  They  are : 


60          Darwin,  and  after  Darwin. 

1.  That   the   theory   of  physiological  selection   is 
opposed  to  the  theory  of  natural  selec' '  >n.     Far  from 
this  being  so,  it  is — at  all  events  in  my  own  opinion — a 
very  important  aid  to  it,  in  preventing  free  intercross- 
ing on  a  common  area,  and  thus  allowing  divergent 
evolution  to  occur  within  that  area. 

2.  That,  in  advancing  the  theory  of  physiological 
selection  as  "  an  additional  suggestion  on  the  origin 
of   species,"    I   wish   to   represent    it   as    being  the 
originating  cause  of  all  species.     What  I  hold  is,  that 
all  species  must  have  owed  their  origin  to  isolation,  in 
some  form  or  other  ;  but  that  as  physiological  selection 
is  only  one  among  many  other  forms  of  isolation  (in- 
cluding natural  selection),  and  as  it  can  only  act  on 
common  areas,  a  large  number  of  species  must  have 
been  formed  without  its  aid. 

3.  That    I    imagine  physiological    varieties  always 
to    arise    "  sporadically,"    or    as    merely    individual 
"  sports  "  of  the  reproductive  system.     On  the  con- 
trary, I  expressly  stated  that  this  is  not  the  way  in 
which    I   suppose   the  "physiological   variation"   to 
arise,  when  giving  origin  to  a  new  species ;  but  that 
it  arises,  whenever   it  is  effectual,  as   a   "  collective 
variation"  affecting  a  number  of  individuals  simul- 
taneously, and  therefore  characterizing  "  a  whole  race, 
or  strain/' 

4.  That  1  suppose  physiological  selection  always  to 
act  alone.    This  I  have  never  supposed.     The  essential 
point  is,  not  that   the  physiological   isolation  is  un- 
associated  with    other   forms   of  isolation,   but   that 
unless  associated  with  some  degree  of  physiological 
isolation,  no   one  of  the  other  forms  is   capable   of 
originating  species  on  common  areas  with  any  approach 


Physiological  Selection.  61 

to  frequency.  This  proposition  is  the  essence  of 
the  new  theory,  and  I  take  it  to  be  proved,  not  only 
by  general  deductive  reasoning  which  shows  that 
it  must  be  so,  but  also  by  the  fact  of  an  otherwise 
inexplicable  association  between  specific  divergence 
on  common  areas  and  some  more  or  less  considerable 
degree  of  mutual  infertility. 


CHAPTER   IV. 
EVIDENCES  OF  PHYSIOLOGICAL  SELECTION. 

I  WILL  now  give  an  outline  sketch  of  the  evidences 
in  favour  of  the  theory  which  has  been  set  forth  in 
the  preceding  chapter,  stating  first  what  is  the  nature 
of  the  verification  which  it  requires. 

The  theory  is  deduced  from  a  highly  general 
association  between  distinctive  specific  characters 
of  any  kind  and  a  relatively  constant  specific 
character  of  a  particular  kind  —  namely,  sexual 
exclusiveness.  For  it  is  from  this  highly  general 
association  that  the  theory  infers  that  this  relatively 
constant  specific  character  has  been  at  least  one  of 
the  needful  conditions  to  the  development  of  the 
other  specific  characters  with  which  it  is  found 
associated.  Hence  the  necessary  verification  must 
begin  by  showing  the  strength  of  the  theory  on  these 
merely  deductive,  or  antecedent,  grounds.  It  may 
then  proceed  to  show  how  far  the  facts  of  organic 
nature  corroborate  the  theory  in  other  and  inde- 
pendent ways. 

First,  let  it  be  carefully  observed  that  here  we  have 
to  do  only  with  the  fact  of  selective  fertility,  and  with 
its  consequences  as  supposed  by  the  theory :  we  have 


Evidences  of  Physiological  Selection.    63 

nothing  to  do  either  with  its  causes  or  its  degrees. 
Not  with  its  causes,  because  in  this  respect  the 
theory  of  physiological  selection  is  in  just  the  same 
position  as  that  of  natural  selection :  it  is  enough  for 
both  if  the  needful  variations  are  provided,  without 
its  being  incumbent  on  either  to  explain  the  causes 
which  produce  them.  Not  with  its  degrees,  because, 
in  the  first  place,  it  can  only  be  those  degrees  of 
variation  which  in  particular  cases  are  supposed 
adequate  to  induce  specific  divergence,  that  fall 
within  the  scope  of  the  theory ;  and  because,  in  the 
second  place,  degrees  which  are  adequate  only  to 
induce — or  to  assist  in  inducing  varietal  divergence, 
must  always  tend  to  increase,  or  pass  into  higher 
degrees. 

Antecedent  Standing  of  the  Theory. 

The  antecedent  standing  or  logical  basis  of  the 
theory  has  already  been  in  large  measure  displayed 
in  the  preceding  chapter ;  for  it  was  impossible  to 
state  the  theory  without  thereby  showing  in  how 
considerable  a  degree  it  is  self-evident.  A  brief 
recapitulation  is  therefore  all  that  is  here  necessary. 

It  has  been  shown  that  divergent  or  polytypic 
evolution  on  common  areas  is  inexplicable  by  natural 
selection  alone.  Hence  the  question  arises:  What 
form  of  isolation  has,  under  such  circumstances, 
rendered  possible  divergent  evolution?  In  answer 
to  this  question  the  theory  of  physiological  selection 
suggests  that  variations  in  the  reproductive  function 
occur  in  such  a  way  as  to  isolate  more  or  less 
perfectly  from  each  other  different  sections  of  a 
species.  While  cross-fertility  remains  unimpaired 


64          Darwin,  and  after  Darwin. 

among  the  members  of  each  section,  there  is  more  or 
less  cross-infertility  when  members  of  either  section 
mate  with  those  of  the  other.  Thus  a  physiological 
barrier  is  interposed  between  the  two  sections  ;  and 
any  divergences  of  structure,  colouring,  or  instinct 
arising  in  the  members  of  either  section  will  not  in 
any  way  be  affected  by  such  divergences  as  arise 
among  the  members  of  the  other. 

In  support  of  this  suggestion,  it  has  been  shown  in 
the  preceding  chapter  that  the  very  general  association 
of  cross-infertility  with  specific  differentiation  points 
most  strongly  to  the  inference  that  the  former  has 
usually  been  an  indispensable  condition  to  the 
occurrence  of  the  latter.  It  cannot  be  denied  that 
in  many  cases  the  specific  distinction  is  now  main- 
tained by  means  of  that  sexual  isolation  which  cross- 
infertility  confers :  it  is  therefore  probable  that  such 
isolation  has  been  instrumental  in  securing  its  initial 
attainment. 

This  probability  is  strengthened  by  the  observed 
fact  that  the  general  association  in  question  is 
conspicuously  absent  in  the  case  of  domesticated 
varieties,  notwithstanding  that  their  multitudinous 
and  diverse  varietal  characters  usually  equal,  and 
frequently  surpass,  specific  characters  in  their  degrees 
of  divergence. 

Since,  then,  it  would  seem  to  be  impossible  for 
divergent  evolution  on  common  areas  to  take  place 
in  the  absence  of  some  mode  of  isolation  ;  since 
cross- infertility  appears  to  be  the  only  possible  mode 
under  the  given  circumstances ;  and  since  among 
domesticated  varieties,  where  isolation  is  otherwise 
secured  by  artificial  means,  cross-infertility  is  usually 


Evidences  of  Physiological  Selection.    65 

absent,  the    logical    foundations   of   the    theory   of 
physiological  selection  would  seem  to  be  securely  laid. 
We  may  therefore  pass  to  more  special  lines  of 
evidence. 

Evidence  from  Geographical  Distribution. 

Darwin  has  adduced  very  good  evidence  to  show 
that  large  areas,  notwithstanding  the  disadvantages 
which  (on  his  theory)  must  arise  from  free  inter- 
crossing, are  what  he  terms  better  manufactories  of 
species  than  smaller  areas,  such  as  oceanic  islands. 
On  the  other  hand,  as  a  matter  of  fact,  oceanic 
islands  are  comparatively  rich  in  peculiar  species. 
These  two  statements,  however,  are  not  incompatible. 
Smaller  areas  are,  as  a  rule,  rich  in  peculiar  species 
relatively  to  the  number  of  their  inhabitants;  but 
it  does  not  follow  that  they  are  rich  in  species  as 
contrasted  with  larger  areas  containing  very  many 
more  inhabitants.  Therefore,  the  rules  are,  that 
large  areas  turn  out  an  absolutely  greater  number 
of  specific  types  than  small  areas ;  although,  relatively 
to  the  number  of  individuals  or  amount  of  population, 
the  small  areas  turn  out  a  larger  number  of  species 
than  the  large  areas. 

Now,  these  two  complementary  rules  admit  of 
being  explained  as  Darwin  explains  them.  Small 
and  isolated  areas  are  rich  in  species  relatively  to 
the  amount  of  population,  because,  as  we  have  before 
seen,  this  population  has  been  permitted  to  develop 
an  independent  history  of  its  own,  shielded  from 
intercrossing  with  parent  forms,  and  from  competition 
with  exotic  forms ;  while,  at  the  same  time,  the 
homogamy  thus  secured,  combined  with  change  of 

HI.  F 


66          Darwin,  and  after  Darwin. 

environment,  will  give  natural  selection  an  improved 
chance  of  finding  new  points  of  departure  for  its 
operation.  On  the  other  hand,  large  and  continuous 
areas  are  favourable  to  the  production  of  numerous 
species,  first,  because  they  contain  a  large  population, 
thus  favouring  the  occurrence  of  numerous  variations  ; 
and,  secondly,  because  the  large  area  furnishes 
a  diversity  of  conditions  in  its  different  parts,  as  to 
food,  climate,  attitude,  &c.,  and  thus  so  many 
different  opportunities  for  the  occurrence  of  sundry 
forms  of  homogamy,  Now,  it  is  obvious  that  of  all 
these  sundry  forms  of  homogamy,  physiological 
selection  must  have  what  may  be  termed  a  first-rate 
opportunity  of  assisting  in  the  manufacture  of  species 
on  large  areas.  For  not  only  is  it  upon  large  and 
continuous  areas  that  the  antagonistic  effects  of 
intercrossing  are  most  pronounced  (and,  therefore, 
that  the  influence  of  physiological  selection  must  be 
most  useful  in  the  work  of  species-making) ;  but  here 
also  the  diversity  in  the  external  conditions  of  life, 
which  the  large  area  supplies  to  different  parts  of 
the  extensive  population,  cannot  fail  to  furnish  physio- 
logical selection  with  a  greater  abundance  of  that 
particular  variation  in  the  reproductive  system  on 
which  its  action  depends.  Again,  and  of  still  more 
importance,  on  large  areas  there  are  a  greater  number 
of  species  already  differentiated  from  one  another 
as  such ;  thus  a  greater  number  of  already  sexually 
differentiated  forms  are  presented  for  further  differen- 
tiation at  the  hands  of  physiological  selection.  For 
all  these  reasons,  therefore,  we  might  have  expected, 
upon  the  new  theory,  that  large  and  continuous  areas 
would  be  good  manufactories  of  species. 


Evidences  of  Physiological  Selection.    67 

Again,  Darwin  has  shown  that  not  only  large 
areas,  but  likewise  "dominant"  genera  within  those 
areas,  are  rich  in  species.  By  dominant  genera  he 
meant  those  which  are  represented  by  numerous 
individuals,  as  compared  with  other  genera  inhabiting 
the  same  area.  This  general  rule  he  explains  by  the 
consideration  that  the  qualities  which  first  led  to  the 
form  being  dominant  must  have  been  useful;  that 
these  would  be  transmitted  to  the  otherwise  varying 
offspring ;  and,  therefore,  that  when  these  offspring 
had  varied  sufficiently  to  become  new  species,  they 
would  still  enjoy  their  ancestral  advantages  in  the 
struggle  for  existence.  And  this,  doubtless,  is  in  part 
a  true  explanation  ;  but  I  also  think  that  the  reason 
why  dominant  genera  are  rich  in  species,  is  chiefly 
because  they  everywhere  present  a  great  number  of 
individuals  exposed  to  relatively  great  differences  in 
tfieir  conditions  of  life :  or.  in  other  words,  that  they 
furnish  the  best  raw  material  for  the  manufacture  of 
species  by  physiological  selection,  as  explained  in 
the  last  paragraph.  For,  if  the  fact  of  dominant 
genera  being  rich  in  species  is  to  be  explained  only 
by  natural  selection,  it  appears  to  me  that  the  useful 
qualities  which  have  already  led  to  the  dominance 
of  the  ancestral  type  ought  rather  to  have  proved 
inimical  to  its  splitting  up  into  a  number  of  sub- 
ordinate types.  If  already  so  far  •'  in  harmony  with 
its  environment "  as  to  have  become  for  this  reason 
dominant,  one  would  suppose  that  there  is  all  the 
more  reason  for  its  not  undergoing  change  by  the 
process  of  natural  selection.  Or,  at  least,  I  do  not 
see  why  the  fact  of  its  being  in  an  unusual  degree 
of  harmony  with  its  environment  should  in  itself 


68          Darwin,  and  after  Darwin. 

constitute  any  unusual  reason  for  its  modification  by 
survival  of  the  fittest.  On  the  other  hand,  as  just 
observed,  I  do  very  plainly  see  why  such  a  reason 
is  furnished  for  the  modifying  influence  of  physio- 
logical selection. 

Let  us  next  turn  to  another  of  Darwin's  general 
rules  with  reference  to  distribution.  He  took  a  great 
deal  of  trouble  to  collect  evidence  of  the  two  following 
facts,  namely,  (i)  that  "species  of  the  larger  genera 
in  each  country  vary  more  frequently  than  the  species 
of  the  smaller  genera  " ;  and  (2)  that  "  many  of  the 
species  included  within  the  larger  genera  resemble 
varieties  in  being  very  closely,  but  unequally,  related 
to  each  other,  and  in  having  restricted  ranges1." 
By  larger  genera  he  means  genera  containing  many 
species ;  and  he  accounts  for  these  general  facts  by 
the  principle,  "  that  where  many  species  of  a  genus 
have  been  formed,  on  an  average  many  are  still 
forming."  But  how  forming  ?  If  we  say  by  natural 
selection  alone,  we  should  expect  to  find  the  multi- 
tudinous species  differing  from  one  another  in  respect 
of  features  presenting  well-marked  adaptive  meanings ; 
yet  this  is  precisely  what  we  do  not  find.  For 
Darwin's  argument  here  is  that  "  in  large  genera  the 
amount  of  difference  between  the  species  is  often 
exceedingly  small,  so  that  in  this  respect  the  species 
of  the  larger  genera  resemble  varieties  more  than  do 
the  species  of  the  smaller  genera."  Therefore  the 
argument,  while  undoubtedly  a  very  forcible  one  in 
favour  of  the  fact  of  evolution,  appears  to  me  scarcely 
consistent  with  the  view  of  this  evolution  being  due 
solely  to  natural  selection.  On  the  other  hand,  the 

1  Origin  of  Species,  pp.  44,  45. 


Evidences  of  Physiological  Selection.    69 

argument  tells  strongly  (though  unconsciously)  in 
favour  of  physiological  selection.  For  the  larger  a 
genus,  or  the  greater  the  number  of  its  species,  the 
greater  must  be  the  opportunity  for  the  occurrence 
of  that  particular  kind  of  variation  on  which  the 
principle  of  physiological  selection  depends.  The 
species  of  a  genus  may  be  regarded  as  so  many 
varieties  which  have  already  been  separated  from  one 
another  physiologically  ;  therefore  each  of  them  may 
now  constitute  a  new  starting-point  for  a  further  and 
similar  separation — particularly  as,  in  virtue  of  their 
previous  segregation,  many  are  now  exposed  to 
different  conditions  of  life.  Thus,  it  seems  to  me, 
we  can  well  understand  why  it  is  that  genera  already 
rich  in  species  tend  to  grow  richer  ;  while  such  is  not 
the  case  in  so  great  a  degree  with  genera  that  are 
poor  in  species.  Moreover,  we  can  well  understand 
that,  multiplication  of  species  being  as  a  rule,  and  in 
the  first  instance,  determined  by  changes  in  the  repro- 
ductive system,  wherever  a  large  number  of  new 
species  are  being  turned  out,  the  secondary  differences 
between  them  should  be  "  often  exceedingly  small " — 
a  general  correlation  which,  so  far  as  I  can  see,  we 
are  not  able  to  understand  on  the  theory  of  natural 
selection. 

The  two  subsidiary  facts,  that  very  closely  allied 
species  have  restricted  ranges,  and  that  dominant 
species  are  rich  in  varieties,  both  seem  to  tell  more 
in  favour  of  physiological  than  of  natural  selection. 
For  "  very  closely  allied  species  "  is  but  another  name 
for  species  which  scarcely  differ  from  one  another 
at  all  except  in  their  reproductive  systems ;  and, 
therefore,  the  more  restricted  their  ranges,  the  more 


70          Darwin,  and  after  Darwin. 

certainly  would  they  have  become  fused  by  inter- 
crossing with  one  another,  had  it  not  been  for  the 
barrier  of  sterility  imposed  by  the  primary  distinc- 
tion. Or  rather,  I  should  say,  had  it  not  been 
for  the  original  occurrence  of  this  barrier,  these  now 
closely-allied  species  could  never  have  become  species. 
Again,  that  dominant  species  should  be  rich  in  varie- 
ties is  what  might  have  been  expected ;  for  the 
greater  the  number  of  individuals  in  a  species,  the 
greater  is  the  chance  of  variations  taking  place  in 
all  parts  of  the  organic  type,  and  particularly  in  the 
reproductive  system,  seeing  that  this  system  is  the 
most  sensitive  to  small  changes  in  the  conditions 
of  life,  and  that  the  greater  the  number  of  indi- 
viduals composing  a  specific  type,  the  more  certainty 
there  is  of  some  of  them  encountering  such 
changes.  Hence,  the  richness  of  dominant  species 
in  varieties  is,  I  believe,  mainly  due  to  the  greater 
opportunity  which  such  species  afford  of  some  degree 
of  cross-infertility  arising  between  their  constituent 
members. 

Here  is  another  general  fact,  also  first  noticed  by 
Darwin,  and  one  which  he  experiences  some  difficulty 
in  explaining  on  the  theory  of  natural  selection.  He 
says : — 

In  travelling  from  north  to  south  over  a  continent,  we  generally 
meet  at  successive  intervals  with  closely-allied  or  representative 
species,  evidently  filling  the  same  place  in  the  economy  of  the 
land.  These  representative  species  often  meet  and  interlock, 
and  as  one  becomes  rarer  and  rarer,  the  other  becomes  more  and 
more  frequent,  till  the  one  replaces  the  other.  But  if  we  com- 
pare these  species  where  they  intermingle,  they  are  generally  as 
absolutely  distinct  from  each  other  in  every  detail  of  structure  as 
are  specimens  taken  from  the  metropolis  of  each.  ...  In  the 


Evidences  of  Physiological  Selection.    71 

intermediate  region,  having  intermediate  conditions  of  life,  why 
do  we  not  now  find  closely-linking  intermediate  varieties  ?  This 
difficulty  for  a  long  time  quite  confounded  me.  But  I  think  it 
can  in  large  part  be  explained 1. 

His  explanation  is  that,  "  as  the  neutral  territory 
between  two  representative  species  is  generally  narrow 
in  comparison  with  the  territory  proper  to  each, 
.  .  .  and  as  varieties  do  not  essentially  differ  from 
species,  the  same  rule  will  probably  apply  to  both  ;  and, 
therefore,  if  we  take  a  varying  species  inhabiting 
a  very  large  area,  we  shall  have  to  adapt  two  varieties 
to  two  large  areas,  and  a  third  variety  to  a  narrow 
intermediate  zone."  It  is  hence  argued  that  this 
third  or  intermediate  variety,  on  account  of  its  existing 
in  lesser  numbers,  will  probably  be  soon  overrun  and 
exterminated  by  the  larger  populations  on  either  side 
of  it.  But  how  is  it  possible  "to  adapt  two  varieties 
to  two  large  areas,  and  a  third  [transitional]  variety 
to  a  narrow  intermediate  zone,"  in  the  face  of  free 
intercrossing  on  a  continuous  area  ?  Let  A,  B,  and 
C  represent  the  three  areas  in  question.  According  to 


A 

B 

C 

the  argument,  variety  A  passes  first  into  variety  B, 
and  then  into  variety  C,  while  variety  B  eventually 
becomes  exterminated  by  the  inroads  both  from 
A  and  C.  But  how  can  all  this  have  taken  place 
with  nothing  to  prevent  intercrossing  throughout  the 
entire  area  A,  B,  Cf  I  confess  that  to  me  it  seems  this 
argument  can  only  hold  on  the  supposition  that  the 
analogy  between  varieties  and  species  extends  to  the 

1  Origin  of  Species,  ed.  6,  pp.  134,  135. 


72          Darwin,  and  after  Darwin. 

reproductive  system ;  or,  in  a  sense  more  absolute 
than  the  argument  has  in  view,  that  "varieties  do 
not  essentially  differ  from  the  species"  which  they 
afterwards  form,  but  from  the  first  show  some 
degree  of  infertility  towards  one  another.  And,  if  so, 
we  have  of  course  to  do  with  the  principles  of  physio- 
logical selection. 

That  in  all  such  cases  of  species-distribution  these 
principles  have  played  an  important  part  in  the 
species-formation,  appears  to  be  rendered  further 
probable  from  the  suddenness  of  transition  on  the 
area  occupied  by  contiguous  species,  as  well  as  from 
the  completeness  of  it — i.  e.  the  absence  of  connecting 
forms.  For  these  facts  combine  to  testify  that  the 
transition  was  originally  due  to  that  particular  change 
in  the  reproductive  systems  of  the  forms  concerned, 
which  still  enables  those  forms  to  "  interlock  "  without 
intercrossing.  On  the  other  hand,  neither  of  these 
facts  appears  to  me  compatible  with  the  theory  of 
species-formation  by  natural  selection  alone. 

But  this  leads  us  to  another  general  fact,  also 
mentioned  by  Darwin,  and  well  recognized  by  all 
naturalists,  namely,  that  closely  allied  species,  or 
species  differing  from  one  another  in  trivial  details, 
usually  occupy  contiguous  areas ;  or,  conversely  stated, 
that  contiguity  of  geographical  position  is  favourable 
to  the  appearance  of  species  closely  allied  to  one 
another.  Now,  the  large  body  of  facts  to  which 
I  here  allude,  but  need  not  at  present  specify,  appear 
to  me  to  constitute  one  of  the  strongest  of  all  my 
arguments  in  favour  of  physiological  selection.  Take, 
for  instance,  a  large  continental  area,  and  follow  across 
it  a  chain  of  species,  each  link  of  which  differs  from 


Evidences  of  Physiological  Selection.     73 

those  on  either  side  of  it  by  the  minute  and  trivial 
distinctions  of  a  secondary  kind,  but  all  the  links 
of  which  differ  from  one  another  in  respect  of  the 
primary  distinction,  so  that  no  one  member  of  the 
series  is  perfectly  fertile  with  any  other  member.  Can 
it  be  supposed  that  in  every  case  this  constant 
primary  distinction  has  been  superinduced  by  the 
secondary  distinctions,  distributed  as  they  are  over 
different  parts  of  all  these  kindred  organisms,  and 
yet  nowhere  presenting  any  but  a  trifling  amount  of 
morphological  change  ? 

For  my  own  part,  I  cannot  believe  —  any  more 
than  Darwin  could  believe — that  all  these  numerous, 
diverse,  and  trivial  changes  have  always  had  the 
accidental  effect  of  inducing  the  same  peculiar  change 
in  the  reproductive  system,  and  so  producing  it  with- 
out any  reference  to  the  process  of  specific  divergence. 
Nor  can  I  believe,  as  Darwin  incidentally  and  pro- 
visionally suggested,  that  prolonged  exposure  to 
uniform  conditions  of  life  have  so  generally  induced 
an  equally  meaningless  result.  I  can  only  believe 
that  all  the  closely  allied  species  inhabiting  our 
supposed  continent,  and  differing  from  one  another 
in  so  many  and  such  divers  points  of  small  detail,  are 
merely  so  many  records  of  the  fact  that  selective 
fertility  has  arisen  among  their  ancestry,  and  has 
thus  given  as  many  opportunities  for  the  occurrence 
of  morphological  differentiations  as  it  has  furnished 
cases  of  efficient  isolation.  Of  course,  I  do  not  deny 
that  many,  or  probably  most,  of  these  trivial  morpho- 
logical differentiations  have  been  produced  by  natural 
selection  on  account  of  their  utility :  I  merely  deny 
that  they  could  have  been  so  produced  on  this 


74          Darwin,  and  after  Darwin. 

common  area,  but  for  the  sexual  isolation  with  which 
every  distinct  set  of  them  is  now  found  to  be  asso- 
ciated. 

Evidence  from  Topographical  Distribution  of 
Species. 

By  topographical  distribution  I  mean  the  distri- 
bution of  organisms  with  reference  to  comparatively 
small  areas,  as  distinguished  from  larger  regions  with 
reference  to  which  the  term  geographical  distribution 
is  appropriate. 

It  will  be  at  once  apparent  that  a  study  of  the 
topographical  distribution  of  organic  types  is  of  even 
more  importance  for  us  than  a  study  of  their  geogra- 
phical distribution.  For  while  the  former  study  is 
conducted,  as  it  were,  with  a  low  power  of  our 
observing  microscope,  the  latter  is  conducted  with 
a  high  power.  The  larger  facts  of  geographical 
distribution  yield,  indeed,  all  the  general  characters 
which  we  might  expect  them  to  yield,  on  the  theory 
that  divergence  of  specific  types  on  common  areas 
has  been  in  chief  part  determined  by  physiological 
conditions.  But  for  the  purpose  of  testing  this 
theory  in  a  still  more  exacting  manner,  it  is  of  the 
first  importance  to  consider  the  more  detailed  facts 
of  topographical  distribution,  since  we  here  come  to 
closer  quarters  with  the  problem  of  specific  differen- 
tiation. Therefore,  as  we  have  already  considered 
this  problem  under  the  most  general  points  of  view, 
we  will  now  consider  it  under  more  special  points 
of  view. 

It  is  self-evident,  as  we  have  seen  in  the  preceding 


Evidences  of  Physiological  Selection.    75 

section,  that  the  greater  the  number  of  individuals 
of  the  same  species  on  a  given  area,  the  less  must 
be  the  power  of  natural  selection  to  split  that  species 
into  two  or  more  allied  types ;  because,  the  more 
crowded  the  population,  the  greater  must  be  the 
uniformitarian  effect  of  free  intercrossing.  This  ob- 
vious fact  has  been  insisted  upon  by  several  previous 
writers  on  Darwinism  ;  and  the  only  reason  why  it 
has  not  been  recognized  by  all  naturalists  is,  that  so 
few  of  them  have  observed  the  all-important  dis- 
tinction between  monotypic  and  polytypic  evolution. 
The  denser  the  population,  and  therefore  the  greater 
the  intercrossing  and  the  severer  the  struggle  for 
existence  within  the  species,  the  better  will  it  be 
for  transmutation  of  the  species  by  natural  selection  ; 
but  the  worse  it  will  be  for  differentiation  of  the 
species  by  this  form  of  homogamy.  On  the  other 
hand,  if  physiological  selection  be  entertained  as 
a  form  of  homogamy,  the  denser  the  population,  the 
better  opportunity  it  will  have  of  differentiating  the 
species,  first,  because  a  greater  number  of  individuals 
will  be  present  in  which  the  physiological  change 
may  arise,  and.  secondly,  because,  if  it  does  arise,  the 
severity  of  the  struggle  for  existence  will  then  give 
natural  selection  a  better  chance  of  acting  rapidly 
and  effectually  on  each  of  the  isolated  sections. 

Hence,  where  the  question  is  whether  selective 
fertility  has  played  any  large  or  general  part  in  the 
differentiation  of  specific  types,  the  best  criterion  we 
can  apply  is  to  ascertain  whether  it  is  a  general 
rule  that  closely  allied  species  occur  in  intimate 
association,  so  that  their  individual  members  con- 
stitute, as  it  were,  a  single  population,  or,  on  the 


j6          Darwin,  and  after  Darwin. 

other  hand,  whether  they  occur  rather  on  different 
sides  of  physical  barriers.  If  they  occur  intimately 
associated,  the  form  of  homogamy  to  which  their 
differentiation  was  due  must  have  presumably  been 
the  physiological  form  ;  whereas,  if  they  are  proved 
to  be  correlated  with  physical  barriers,  the  form  of 
homogamy  which  was  concerned  in  their  differen- 
tiation must  presumably  have  been  the  geographical 
form. 

Now,  at  first  this  consideration  was  a  trouble  to 
me,  because  Moritz  Wagner  had  strenuously  argued 
—and  supported  his  argument  by  a  considerable 
wealth  of  illustration — that  allied  species  are  always 
found  correlated  with  physical  barriers  or  discon- 
tinuous areas.  Weismann's  answer,  indeed,  had 
shown  that  Wagner's  statement  was  much  too  general : 
nevertheless,  I  was  disappointed  to  find  that  so 
much  could  be  said  in  favour  of  the  geographical 
(or  topographical)  form  of  isolation  where  closely 
allied  species  are  concerned.  Subsequently,  however. 
I  read  the  writings  of  Nageli  on  this  subject,  and 
in  them  I  find  a  very  different  state  of  matters 
represented. 

Seeing  as  clearly  as  Wagner  that  it  is  impossible 
under  any  circumstances  for  natural  selection  to 
cause  specific  differentiation  unless  assisted  by  some 
other  forms  of  homogamy,  but  committing  the  same 
oversight  as  Wagner  and  Weismann  in  supposing 
that  the  only  other  form  of  homogamy  in  nature  is 
geographical  isolation,  Nageli,  with  great  force  of 
reasoning,  and  by  many  examples,  founded  his  argu- 
ment against  the  theory  of  natural  selection  on  the 
ground  that  in  the  vegetable  kingdom  closely  allied 


Evidences  of  Physiological  Selection.    77 

species  are  most  frequently  found  in  intimate  asso- 
ciation with  one  another,  not,  that  is  to  say.  in  any 
way  isolated  by  means  of  physical  barriers.  This 
argument  is  everywhere  logically  intact;  and.  as  he 
sustains  it  by  a  large  knowledge  of  topographical 
botany,  his  indictment  against  natural  selection  as 
a  cause  of  specific  differentiation  appeared  to  be 
insurmountable.  And,  in  point  of  fact,  it  was  in- 
surmountable;  so  that  the  whole  problem  of  the 
origin  of  species  by  differentiation  on  common  areas 
has  hitherto  been  left  in  utter  obscurity.  Nor  is  there 
now  any  escape  from  this  obscurity,  unless  we  enter- 
tain the  "  supplementary  factor  "  of  selective  fertility. 
And,  apparently,  the  only  reason  why  this  has  not 
been  universally  recognized,  is  because  Darwinians 
have  hitherto  failed  to  perceive  the  greatness  of  the 
distinction  between  the  differentiation  and  the  trans- 
mutation of  species ;  and  hence  have  habitually  met 
such  overwhelming  difficulties  as  Nageli  presented  by 
an  illogical  confounding  of  these  two  totally  distinct 
things. 

But  if  the  idea  of  selective  fertility  had  ever 
occurred  to  Nageli  as  a  form  of  segregation  which 
gives  rise  to  specific  differentiation,  I  can  have  no 
doubt  that  so  astute  and  logical  a  thinker  would 
have  perceived  that  his  whole  indictment  against 
natural  selection  was  answered.  For  it  is  incredible 
that  he  should  not  have  perceived  how  this  physio- 
logical form  of  homogamy  (supposing  it  to  arise  before 
or  during,  and  not  after  the  specific  differentiation) 
would  perform  exactly  the  same  function  on  a  con- 
tinuous area,  as  he  allowed  that  "  isolation  "  does  on 
a  discontinuous  one. 


78          Darwin,  and  after  Darwin. 

However,  be  this  as  it  may,  there  cannot  be  any 
question  touching  the  immense  value  of  his  facts  and 
arguments  as  evidence  in  favour  of  physiological 
selection  —  albeit  this  evidence  was  given  uncon- 
sciously, or,  as  it  were,  prophetically.  Therefore 
I  will  here  quote  a  few  examples  of  both,  from  his 
paper  Du  Dfoeloppement  des  Espfaes  Societies *. 

After  stating  the  theory  of  natural  selection,  he 
says  that  if  the  theory  is  (of  itself)  a  true  explanation 
of  the  origin  (or  divergence)  of  specific  forms,  it 
ought  to  follow  that 

two  closely  allied  forms,  derived  the  one  from  the  other, 
would  necessarily  occupy  two  different  geographical  areas  [or 
topographical  stations],  since  otherwise  they  would  soon  become 
blended.  Until  they  had  already  become  sufficiently  consolidated 
as  distinct  species  to  render  mutual  intercrossing  highly  impro- 
bable, they  could  not  be  intermingled  without  disadvantage 
[to  differentiation].  Had  Darwin  endeavoured  to  support  his 
hypothesis  by  facts,  he  would,  at  least  in  the  vegetable  kingdom, 
have  found  little  to  favour  his  cause.  I  can  cite  many  hundreds 
of  cases,  in  which  species  in  every  stage  of  development  have 
been  found  closely  mingling  with  one  another,  and  not  in  any 
way  isolated.  Therefore,  I  do  not  think  that  one  can  rightly 
speak  of  natural  selection  in  the  Darwinian  sense  in  the 
vegetable  kingdom;  and,  in  my  estimation,  there  is  a  great 
difference  between  the  formation  of  species  by  nature  and  the 
production  of  stock  by  a  breeder.  ...  (p.  212). 

Of  the  two  kinds  of  distribution  (i.  e.  growing  apart  and 
growing  together),  Synoicy  (or  growing  together)  is  by  far 
the  most  usual  in  nature.  I  reckon  that  out  of  a  hundred 
allied  vegetable  forms,  at  least  ninety-five  would  be  found  to  be 
synoical  (p.  219). 

This  is  a  most  important  point.    That  so  enormous 

1  Archives  des  Sciences  physiques  etnatnreilcs  \(  leneve),  vol.  liii.  (1875), 
pp.  211-236. 


Evidences  of  Physiological  Selection.     79 

a  proportion  of  vegetable  species  should  have  origi- 
nated in  intimate  association  with  their  parent  or 
sister  types,  is  clearly  unintelligible  on  the  theory  of 
natural  selection  alone ;  there  obviously  must  be  some 
other  form  of  homogamy  which,  whether  or  not  in 
all  places  associated  with  natural  selection,  is  the 
primary  condition  to  the  differentiation.  Such, 
I  hold  with  Nageli,  is  a  logical  necessity ;  and  this 
whether  or  not  I  am  right  in  believing  the  other 
form  of  homogamy  in  question  to  be  selective  fertility. 
But  I  go  further  and  say,  Surely  there  can  be  no 
rational  question  that  this  other  form  of  homogamy 
must  have  been,  at  any  rate  as  a  highly  general  rule, 
the  one  which  I  have  assigned.  For  how  is  it  that 
in  these  ninety-five  per  cent,  of  cases,  where  vegetable 
species  are  growing  intimately  associated  with  their 
nearest  allies,  there  is  no  hybridizing,  or  blending 
and  relapsing  to  the  original  undifferentiated  types? 
We  know  well  the  answer.  These  are  fully  differen- 
tiated species,  and,  as  such,  are  protected  from  mutual 
intercrossing  by  the  barrier  of  mutual  sterility.  But 
now,  if  this  bar  is  thus  necessary  for  preserving  the 
specific  distinctions  when  they  have  been  fully 
developed,  much  more  must  it  have  been  so  to  admit 
of  their  development ;  or,  otherwise  stated,  since  we 
know  that  this  barrier  is  associated  with  "  synoical " 
species,  and  since  we  clearly  perceive  that  were  it 
withdrawn  these  species  would  soon  cease  to  exist, 
can  we  reasonably  doubt  that  their  existence  (or 
origin)  is  due  to  the  previous  erection  of  this 
barrier?  If  synoical  species  were  comparatively 
rare,  the  validity  of  such  reasoning  might  be  open 
to  question  ;  or,  even  if  we  should  not  doubt  it  in 


8o  Darwin,  and  after  Darwin. 

such  cases,  at  any  rate  we  might  well  doubt  the 
importance  or  extent  of  selective  fertility  as  a  factor 
in  the  origination  of  species.  But  the  value  of 
Nageli's  writings  on  the  present  subject  consists  in 
showing  that  synoical  species  constitute  so  over- 
whelming a  majority  of  the  vegetable  kingdom,  that 
here,  at  all  events,  it  appears  impossible  to  rate  too 
highly  the  importance  of  the  principle  I  have  called 
physiological  selection. 


CHAPTER  V. 

FURTHER  EVIDENCES  OF  PHYSIOLOGICAL 
SELECTION. 

Evidence  from  Topographical  Distribution  of 
Varieties. 

IN  the  last  section  we  have  considered  the  topo- 
graphical distribution  of  closely  allied  species.  I  now 
propose  to  go  still  further  into  matters  of  detail,  by 
considering  the  case  of  natural  varieties.  And  here 
we  come  upon  a  branch  of  our  inquiry  where  we  may 
well  expect  to  meet  with  the  most  crucial  tests  of 
our  theory.  For  if  it  should  appear  that  these  nascent 
species  more  or  less  resemble  fully  developed  species 
in  presenting  the  feature  of  cross-infertility,  the  theory 
would  be  verified  in  the  most  direct  and  conclusive 
manner  possible.  These  nascent  species  may  be 
called  embryo  species,  which  are  actually  in  course 
of  differentiation  from  their  parent-type ;  and  there- 
fore, if  they  do  not  exhibit  the  feature  in  relation 
to  that  type  which  the  present  theory  infers  to  be 
necessary  for  the  purposes  of  differentiation,  the 
theory  must  be  abandoned.  On  the  other  hand,  if 
they  do  exhibit  this  feature,  it  is  just  the  feature 
which  the  theory  predicted  as  one  that  would  be 
found  highly  characteristic  of  such  embryo  types. 


82  Darwin,  and  after  Darwin. 

Contrariwise,  the  theory  of  natural  selection  can  have 
no  reason  to  form  any  such  anticipation  ;  or  rather 
its  anticipation  would  necessarily  require  to  be  the 
exact  opposite.  For,  according  to  this  theory,  the 
cross-infertility  of  allied  species  is  due,  either  to 
correlation  with  morphological  changes  which  are 
being  produced  by  the  selection,  or  else,  as  Darwin 
supposed,  to  "  prolonged  exposure  to  uniform  con- 
ditions of  life  "  ;  and  thus,  in  either  case,  the  sterility 
variation  ought  to  be,  as  a  general  rule  at  all  events, 
subsequent  to  the  specific  differentiation,  and,  ac- 
cording to  Darwin's  view,  long  subsequent.  Thus 
we  ought  not  to  find  that  the  physiological  change 
is  ever,  on  any  large  or  general  scale,  the  initial 
change ;  nor  ought  we  to  find  that  it  is,  on  any 
such  scale,  even  so  much  as  a  contemporary  change : 
there  ought,  in  fact,  to  be  no  constant  or  habitual 
association  between  divergence  of  embryo-types  and 
the  concurrence  of  cross-infertility. 

Now,  it  will  be  my  endeavour  to  prove  that 
there  is  an  extraordinarily  general  association  between 
varietal  divergence  and  cross-infertility,  wherever 
common  areas  are  concerned;  and  in  as  far  as  this 
can  be  proved,  I  take  it  that  the  evidence  will  make 
wholly  in  favour  of  physiological  selection  as  the 
prime  condition  to  specific  divergence,  while  at  the 
same  time  they  will  make  no  less  wholly,  and  quite 
independently,  against  natural  selection  as  the  unaided 
cause  of  such  divergence. 

I  shall  begin  with  some  further  quotations  from 
Nageli. 

Species  may  be  synoical  at  all  stages  of  relationship.  We 
come  across  varieties,  scarcely  distinguishable  from  one  another, 


Evidences  of  Physiological  Selection.    83 

growing  in  the  same  locality  (as,  for  example,  the  Cirsium 
heterophyllum,  with  smooth  or  jagged  leaves,  the  Hieracium 
sylvaticum,  with  or  without  caulinary  leaves) ;  again,  we  meet 
other  varieties  more  accentuated  (as  the  H.  hoppeanum,  with 
under  ligules  of  white  or  red,  the  Campanula,  with  white  or  lilac 
flowers,  &c.),  other  varieties  even  more  marked,  which  might 
almost  be  elevated  to  the  rank  of  species  {Hieracium  alpinum, 
with  hairs  and  glands,  and  the  new  form  H.  holadenium,  which 
has  only  glands,  Campanula  rotundifolia  with  smooth  and  hairy 
leaves),  or  forms  still  more  distinct,  up  to  well-defined  species. 
I  could  enumerate  endless  examples  at  all  stages. 

It  will  be  seen  that  in  my  definition  of  synoicy  I  do  not  mean 
to  assert  that  all  allied  forms  are  invariably  found  together,  but 
that  they  are  much  more  often  seen  in  groups  than  singly. 
Take,  for  instance,  nine  forms  closely  related  (A  to  /).  A,  E,  H 
will  be  found  side  by  side  at  one  point,  £,  D  at  another,  C,  F 
at  a  third,  &c.  These  facts  are  plainly  opposed  to  the  theory  of 
isolation  and  amixin,  and  make,  on  the  contrary,  in  favour  of  the 
social  development  of  species  doc.  cit.,  p.  221). 

Not  to  multiply  quotations  to  the  same  general  effect, 
I  will  supply  but  one  other,  referring  to  a  particular 
case. 

At  one  spot  (Rothivand)  much  exposed  to  the  sun,  and 
difficult  of  access,  I  remarked  two  closely  allied  forms,  so  nearly 
related  to  H.  villosum  that  this  would  seem  to  be  an  interme- 
diary form  between  the  two.  One  of  these  (H.  •villosissimum} 
is  distinguished  by  its  tongue  and  thick  pubescence,  its  tolerably 
large  capitula,  and  by  the  lengthened  and  separated  scales  of 
the  involucrum ;  the  other,  on  the  contrary  (H.  elongatum\  is 
less  pubescent,  has  smaller  capitula,  and  more  compact  scales 
on  the  involucrum  than  H.  villosum.  Both  are  finally  distin- 
guishable from  the  type  by  their  longer  stalks,  which  are  more 
decidedly  aphyllous,  and  by  their  later  flowering.  At  the  spot 
where  I  found  them  the  two  forms  were  closely  intermingled, 
and  each  was  represented  by  a  considerable  number  of  plants. 
I  did  not  find  them  anywhere  else  on  the  mountain,  nor  could 
I  find  at  the  spot  where  these  were  growing  a  single  specimen 
of  the  true  H.  inllosvm,  nor  a  single  hybrid  from  these  two. 
G  2 


84          Darwin,  and  after  Darwin. 

I  concluded  that  these  two  new  forms  had,  by  joining  their 
forces,  expelled  the  H.  mllosum  from  its  primitive  abode,  but 
had  not  succeeded  in  displacing  one  another.  As  to  their  origin, 
they  had  evidently  developed  in  two  different  directions  from 
a  common  point  of  departure,  namely  H.  mllosum.  They  had 
succeeded,  not  only  in  separating  themselves  from  the  original 
form,  but  also  in  preventing  any  intermediary  form  from  inter- 
posing. I  thought  myself  therefore  justified  in  considering  this 
as  a  case  of  varieties  which  have  come  into  existence  subsequently 
to  the  Glacial  epoch.  The  morphological  characteristics  of  the 
three  forms  are  sufficiently  distinct  for  them  to  be  designated  as 
species  by  a  good  many  writers.  They  are  better  defined  than 
some  of  MM.  Frolich  and  Fries'  weaker  species,  and  as  well 
defined  as  some  of  MM.  Koch  and  Grisebach's  (p.  222). 

Now  it  is  clear,  without  comment,  that  all  this  is 
exactly  as  it  ought  to  be,  if  allied  species  have  been 
differentiated  on  common  areas  by  selective  fertility. 
For  if,  as  Nageli  elsewhere  says,  "one  meets  forms 
in  nature  associated  with  one  another,  and  severally 
distinguished  by  every  possible  degree  of  differen- 
tiation," not  only  as  Nageli  adds,  does  this  general 
fact  lead  to  the  inference  that  species  are  (usually) 
developed  when  plants  grow  intimately  associated 
together;  but  as  certainly  it  leads  to  the  further 
inference  that  such  development  must  be  due  to 
a  prior  development  of  cross-infertility  between  the 
diverging  varietal  forms,  cross-infertility  which  is 
therefore  afterwards  so  characteristic  of  the  allied 
species,  when  these  are  found,  in  their  fully  dif- 
ferentiated condition,  still  occupying  the  same  area 
in  large  and  intimately  mingled  populations, 

To  my  mind  there  could  not  be  any  inference  more 
strongly  grounded  than  this,  because,  with  the  one 
exception  of  the  physiological  form,  no  other  form 
of  homogamy  can  be  conceived  which  shall  account 


Evidences  of  Physiological  Selection.    85 

for  the  origin  and  permanence  of  these  synoical 
varieties,  in  all  degrees  of  differentiation  up  to  well- 
defined  synoical  species.  Least  of  all,  as  we  have 
seen,  can  natural  selection  alone  have  had  anything 
to  do  with  such  a  state  of  matters ;  while,  as  we  have 
likewise  seen,  in  all  its  details  it  is  exactly  the  state 
of  matters  which  the  theory  of  physiological  selection 
requires. 

Nevertheless,  although  this  inference  is  so  strongly 
grounded,  we  ought  to  remember  that  it  is  only  an 
inference.  In  order  fully  to  verify  the  theory  of 
physiological  selection,  we  ought  to  prove  by  experi- 
ment the  fact  of  cross-infertility  between  these  synoical 
varieties;  as  we  learn  that  it  afterwards  obtains  between 
synoical  species.  It  is  to  be  regretted  that  the  theory 
of  physiological  selection  did  not  occur  to  the  mind 
of  Nageli,  because  he  would  then,  no  doubt,  have 
ascertained  this  by  actual  experiment.  As  it  is,  the 
great  value  of  his  observations  goes  no  further  than 
establishing  a  strong  presumption,  that  it  must  be 
selective  fertility  which  causes  the  progressive  dif 
ferentiation  of  synoical  varieties;  and  also  that,  if 
so,  this  must  be  the  principal  factor  in  the  differentia- 
tion of  vegetable  species,  seeing  that  some  ninety-five 
per  cent,  are  of  synoical  origin. 

Evidence  from  Experimental  Research. 

My  paper  on  Physiological  Selection  pointed  out  that 
the  whole  theory  would  have  to  stand  or  fall  with  the 
experimental  proof  of  the  presence  or  the  absence  of 
cross-infertility  between  varieties  of  the  same  species 
growing  on  common  areas.  From  the  facts  and 
considerations  which  we  have  hitherto  been  dealing 


86          Darwin,  and  after  Danvin. 

with,  it  did  indeed  appear  to  me  that  there  was  the 
strongest  conceivable  ground  for  inferring  that  cross- 
infertility  between  such  varieties  would  be  found  by 
experiment  to  be  a  phenomenon  of  highly  gene- 
ral occurrence— amply  sufficient  ground  to  prove 
that  allied  species  on  common  areas  for  the  most  part 
owed  their  origin  to  this  character  of  mutual  sterility, 
and  not  vice  versa  as  previously  supposed.  At  that 
time  I  was  not  aware  that  any  experiments  had  been 
made  in  this  direction.  Soon  after  the  paper  was 
published,  however,  my  attention  was  directed  to  a 
laborious  research  which  had  been  directed  to  this 
very  point,  and  carried  on  for  more  than  thirty  years, 
by  M.  Jordan  *.  This  had  not  attracted  the  general 
notice  which  it  undoubtedly  deserved ;  and  I  have 
since  ascertained  that  even  Darwin  began  to  look 
into  it  only  a  few  months  before  his  death. 

Having  devoted  his  life  to  closely  observing  in 
divers  stations  multitudes  of  different  species  of  plants 
— annuals  and  perennials,  bulbous  and  aquatic,  trees 
and  shrubs  —  M.  Jordan  has  been  able  to  satisfy  him- 
self, and  the  French  school  of  botanists  to  which  this 
line  of  observation  has  given  rise,  that  in  most  cases 
(or  "nearly  everywhere"),  when  a  Linnean  species 
is  indigenous  to  a  country  and  is  there  of  common 
occurrence,  this  species  within  that  district  is  repre- 
sented by  more  or  less  numerous  and  perfectly  constant 
varieties.  These  varieties  are  constituted  by  such 
minute  differences  of  morphological  character  that 

1  Remarques  sur  le  fait  de  F  existence  en  socit. <ti  ft  Fitat  sanvage  des 
especes  vegetales  affines  et  sur  d'autres  faits  relatifs  h  la  question  de 
fesftce,  par  Alexis  Jordan  ;  lues  au  congres  de  1'Association  Fian9aise 
pour  1' Avancement  des  Sciences,  am*  session,  Lyon,  seance  de  28  Aout, 
1873. 


Evidences  of  Physiological  Selection.    87 

their  very  existence  eluded  the  observation  of  botanists, 
until  M.  Jordan  began  to  search  specially  for  them  as 
the  special  objects  of  his  scrutiny.  Moreover,  these 
varieties  of  a  Linnean  species  occupy  common  areas, 
and  there  grow  in  intimate  association  with  one 
another,  or  as  M.  Jordan  says,  '; pele-mele"  So  far, 
be  it  noticed,  Jordan  was  proceeding  on  exactly  the 
same  lines  as  Nageli ;  only  he  carried  his  observa- 
tions over  a  still  wider  range  of  species  on  the  one 
hand,  and  into  a  still  minuter  search  for  varieties 
on  the  other.  But  the  all-important  point  for  us  is, 
that  he  further  proceeded  to  test  by  experiment  the 
physiological  relations  between  these  morphological 
varieties  ;  and  found,  in  many  hundreds  of  cases, 
that  they  not  only  came  true  to  seed  (i.  e.  are  hereditary 
and  not  merely  climatic),  but  likewise  cross-sterile 
inter  se.  For  these  reasons,  M.  Jordan,  who  is 
opposed  to  the  theory  of  evolution,  regards  all  such 
varieties  as  separately  created  species ;  and  the 
inspiring  motive  of  his  prolonged  investigations  has 
been  a  desire  to  multiply  these  proofs  of  creative 
energy.  But  it  clearly  makes  no  difference,  so  far 
as  evolutionists  are  concerned  with  them,  whether 
all  this  multitude  of  sexually  isolated  forms  be  de- 
nominated species  or  varieties. 

The  points  which  are  of  importance  to  evolu- 
tionists— and  of  the  first  order  of  importance  in  the 
present  connexion — may  be  briefly  summarized  as 
follows : — 

(i)  The  research  embraces  large  numbers  of  species, 
belonging  to  very  numerous  and  very  varied  orders 
of  plants ;  (2)  in  the  majority  of  cases — although  not 
all — indigenous  species  which  are  of  common  occur- 


88          Darwin,  and  after  Darwin, 

rence  present  constant  varieties  ;  (3)  these  varieties, 
nevertheless,  may  be  morphologically  so  slight  as  to 
be  almost  imperceptible;  (4)  they  occupy  common 
areas  and  grow  in  intimate  association  ;  (5)  although 
many  of  them  have  undergone  so  small  an  amount 
of  morphological  change,  they  have  undergone  a  sur- 
prising amount  of  physiological  change ;  for  (6)  not 
only  do  very  many  of  these  varieties  come  true  to 
seed  ;  but,  (7)  when  they  do,  they  are  always  more  or 
less  cross- infertile  inter  se. 

Now,  it  is  self-evident  that  every  one  of  these  seven 
points  is  exactly  what  the  theory  of  physiological 
selection  requires,  while  there  is  not  one  of  them 
which  it  does  not  require.  For  if  the  theory  be 
sound,  we  should  expect  to  find  large  numbers  of 
species  belonging  to  numerous  and  varied  orders 
of  plants  presenting  constant  varieties  on  common 
areas ;  we  should  expect  this  to  be  a  highly  general, 
though  not  a  universal,  rule ;  and  we  should  expect 
it  to  apply  only  to  species  which  are  indigenous.  More- 
over, we  should  expect  these  varieties,  although  but 
slightly  differentiated  morphologically,  to  present  a 
great  differentiation  physiologically— and  this  in  the 
special  direction  of  selective  fertility,  combined,  of 
course,  with  heredity. 

On  the  other  hand,  as  I  have  said,  this  catalogue 
of  evidences  leaves  nothing  to  be  supplied.  It  gives 
us  all  the  facts — and  no  more  than  all  the  facts — 
which  my  paper  on  Physiological  Selection  anticipated 
as  the  eventual  result  of  a  prolonged  experimental 
research.  And  if  I  have  to  regret  my  ignorance  of 
these  facts  when  that  paper  was  published,  at  any 
rate  it  now  furnishes  the  best  proof  that  my  anticipa- 


Evidences  of  Physiological  Selection.    89 

tions  were  not  guided  by  the  results  of  a  verification 
which  had  already  been  supplied.  These  anticipations 
were  deduced  exclusively  from  the  theory  itself,  as 
representing  what  ought  to  be  the  case  if  the  theory 
were  true ;  and,  I  must  confess,  if  I  had  then  been 
told  that  they  had  already  been  realized— that  it 
had  actually  been  found  to  be  a  general  rule  that 
endemic  species  present  constant  and  hereditary 
varieties,  intimately  commingled  on  common  areas, 
morphologically  almost  indistinguishable,  but  physio- 
logically isolated  by  selective  fertility — I  should 
have  felt  that  the  theory  had  been  verified  in 
advance.  For  there  are  only  two  alternatives: 
either  these  things  are  due  to  physiological  selection, 
or  else  they  are  due — as  M.  Jordan  himself  believes 
— to  special  creation.  Which  is  equivalent  to  say- 
ing that,  for  evolutionists,  the  facts  must  be  held 
to  verify  the  former  theory  in  as  complete  a  manner 
as  it  is  logically  possible  for  the  theory  to  be 
verified. 

Evidence  from  Prepotency. 

We  have  now  to  consider  the  bearing  of  what  is 
called  '•  prepotency "  on  the  theory  of  physiological 
selection. 

Speaking  of  the  vast  number  of  species  of  Com- 
positae,  Darwin  says  : — 

There  can  be  no  doubt  that  if  the  pollen  of  all  these  species 
could  be  simultaneously  or  successively  placed  on  the  stigma  of 
any  one  species,  this  one  would  elect  with  unerring  certainty  its 
own  pollen.  This  elective  capacity  is  all  the  more  wonderful,  as 
it  must  have  been  acquired  since  the  many  species  of  this  great 
group  of  plants  branched  off  from  a  common  progenitor. 


90  Darwin,  and  after  Darwin. 

Darwin  is  here  speaking  of  elective  affinity  in 
its  fully  developed  form,  as  absolute  cross-sterility 
between  fully  differentiated  species.  But  we  meet 
with  all  lower  degrees  of  cross-infertility—sometimes 
between  "  incipient  species,"  or  permanent  varieties, 
and  at  other  times  between  closely  allied  species. 
It  is  then  known  as  "prepotency"  of  the  pollen 
belonging  to  the  same  variety  or  species  over  the 
pollen  of  the  other  variety  or  species,  when  both  sets 
of  pollen  are  applied  to  the  same  stigma.  Although 
in  the  absence  of  the  prepotent  pollen  the  less  potent 
will  fertilize  the  seed,  yet,  such  is  the  appetency  for 
the  more  appropriate  pollen,  that  even  if  this  be 
applied  to  the  stigma  some  considerable  time  after 
the  other,  it  will  outstrip  or  overcome  the  other  in 
fertilizing  the  ovules,  and  therefore  produce  the 
same  result  on  the  next  generation  as  if  it  had  been 
applied  to  the  mother  plant  without  any  admixture 
of  the  less  potent  pollen,  although  in  some  cases  such 
incipient  degrees  of  cross-infertility  are  further  shown 
by  the  number  or  quality  of  the  seeds  being  fewer 
or  inferior. 

Now,  in  different  varieties  and  in  different  allied 
species,  all  degrees  of  such  prepotency  have  been 
noticed  by  many  observers,  from  the  faintest  per- 
ceptible amount  up  to  complete  impotency  of  the 
alien  pollen — when,  of  course,  there  is  absolute 
sterility  between  the  two  varieties  or  allied  species. 
The  inference  is  obvious.  In  this  graduated  scale 
of  prepotency— beginning  with  an  experimentally 
almost  imperceptible  amount  of  sexual  differentia- 
tion between  two  varieties,  and  ending  in  an  absolute 
partitioning  of  two  allied  species — we  have  the  only 


Evidences  of  Physiological  Selection.    91 

remaining  fact  that  is  required  to  complete  the  case 
in  favour  of  the  present  theory.  We  are  here  brought 
back  to  the  very  earliest  stages  of  physiological  differ- 
entiation or  to  the  stages  which  lie  behind  Jordan's 
"  Physiological  Species  "  ;  and  therefore,  when  taken 
in  conjunction  with  his  results,  the  phenomena  01 
prepotency  may  be  said  to  give  us  the  complete  and 
final  demonstration  of  one  continuous  development, 
which,  beginning  in  an  almost  imperceptible  amount 
of  cross-infertility,  ends  in  absolute  cross-sterility. 
The  "  elective  capacity "  to  which  Darwin  alludes  as 
having  been  "  acquired "  by  all  the  species  of  Com- 
positae  since  they  "branched  off  from  a  common 
progenitor,"  is  thus  seen  among  innumerable  other 
species  actually  in  process  of  acquisition ;  and  so 
we  can  perfectly  well  understand,  what  is  otherwise 
unintelligible,  that  closely  allied  species  oi  plants 
occur,  in  ninety-five  per  cent,  of  cases,  intimately  asso- 
ciated on  common  areas,  while  exhibiting  towards  one 
another  the  character  of  mutual  sterility. 

But  more  than  this.  The  importance  of  the  wide- 
spread phenomena  of  prepotency  to  the  theory  of 
physiological  selection  does  not  consist  merely  in 
thus  supplying  the  last  link  in  the  chain  of  evidence 
touching  the  origin  of  species  by  selective  fertility, 
or  "elective  capacity."  These  phenomena  are  of 
further  importance  as  showing  how  in  plants,  at  all 
events,  physiological  selection  appears  to  be  frequently 
capable  of  differentiating  specific  types  without  the 
necessary  assistance  of  any  other  form  of  homogamy. 
In  my  original  statement  of  the  theory,  I  was  careful 
to  insist  upon  the  great  value,  as  differentiating  agents, 
of  even  small  degrees  of  other  forms  of  homogamy 


92          Darwin ,  and  after  Darwin. 

when  co-operating  with  physiological  selection.  But 
I  also  stated  my  belief  that  in  many  cases  selective 
fertility  is  presumably  of  itself  capable  of  splitting 
a  specific  type ;  and  the  reason  why  I  still  believe 
this  is,  that  I  do  not  otherwise  understand  these  pheno- 
mena of  prepotency.  I  cannot  believe  that  in  all  the 
innumerable  cases  where  they  arise,  they  have  been 
super-induced  by  some  prior  morphological  changes 
going  on  in  some  other  part  of  the  organism,  or  by 
"prolonged  exposure  to  uniform  conditions  of  life," 
on  the  part  of  two  wellnigh  identical  forms  which 
have  arisen  intimately  commingled  in  exactly  the 
same  environment,  and  under  the  operation  of  a  pre- 
viously universal  intercrossing.  Even  if  such  a  thing 
could  be  imagined  as  happening  occasionally,  I  feel 
it  difficult  to  imagine  that  it  can  happen  habitually, 
and  yet  this  view  must  be  held  by  those  who  would 
attribute  prepotency  to  natural  selection. 

It  must  never  be  forgotten  that  the  relatively 
enormous  changes  as  to  size,  structure,  habit,  &c., 
which  are  presented  by  our  domesticated  plants  as 
results  of  artificial  selection,  do  not  entail  the  physio- 
logical character  of  cross-sterility  in  any  degree, 
save  possibly  in  some  small  number  of  cases.  Although 
in  wild  species  any  correspondingly  small  percentage 
of  cases  (where  natural  selection  happens  to  hit  upon 
parts  of  the  organism  modifications  of  which  produce 
the  physiological  change  by  way  of  correlation)  would 
doubtless  be  the  ones  to  survive  on  common  areas, 
still  it  is  surely  incredible  that  such  an  accidental 
association  between  natural  selection  and  cross- 
infertility  is  so  habitually  the  means  of  specific 
differentiation  as  the  facts  of  prepotency  (together 


Evidences  of  Physiological  Selection.    93 

with  the  observations  of  Jordan  and  Nageli)  would 
necessarily  demand. 

Moreover,  this  view  of  the  matter  is  still  lurther 
corroborated  by  certain  other  facts  and  considerations. 
For  example,  the  phenomena  of  prepotency  (whether 
as  between  varieties  or  between  closely  allied  species) 
are  found  to  occur  when  the  two  forms  occupy  a 
common  area,  i.e.  are  growing  intermingled  with 
one  another.  Therefore,  but  for  this  physiological 
differentiation,  there  could  be  absolutely  nothing  to 
prevent  free  intercrossing.  Yet  the  fact  that  hybrids 
are  so  comparatively  rare  in  a  state  of  nature— a  fact 
which  Sir  Joseph  Hooker  has  pointed  out  to  me  as 
otherwise  inexplicable —proves  the  efficacy  of  even 
a  low  degree  of  such  differentiation  in  preventing 
the  physiologically-differentiated  forms  from  inter- 
crossing. Even  in  cases  where  there  is  no  difficulty 
in  producing  artificial  hybrids  or  mongrels  between 
species  or  varieties  growing  on  common  areas,  it  is 
perfectly  astonishing  what  an  extremely  small  per- 
centage of  the  hybrid  or  mongrel  forms  are  found  to 
occur  in  nature.  And  there  can  be  no  question  that 
this  is  due  to  the  very  efficient  manner  in  which 
prepotency  does  its  work— efficient,  I  mean,  from 
the  point  of  view  of  the  new  theory ;  for  upon  any 
other  theory  prepotency  is  a  meaningless  pheno- 
menon, which,  notwithstanding  its  frequent  occur- 
rence, plays  no  part  whatever  in  the  process  of  organic 
evolution. 

I  attach  considerable  importance  to  the  phenomena 
of  prepotency  in  view  of  the  contrast  which  is  pre- 
sented between  plants  and  animals  in  the  relation  of 
their  species  to  physical  barriers.  For  animals— 


94          Darwin,  and  after  Danvin. 

and  especially  the  higher  animals — appear  to  depend 
for  their  specific  differentiations  upon  such  barriers 
much  more  than  in  the  case  with  plants.  This  is  no 
more  than  we  should  expect ;  for,  in  accordance  with 
our  theory,  selective  fertility  is  not  so  likely  to  work 
alone  in  the  case  of  the  higher  animals  which  mate 
together,  as  in  plants  which  are  fertilized  through  the 
agency  of  wind  or  insects.  In  the  former  case  there 
is  no  opportunity  given  for  the  first  rise  of  cross- 
infertility,  in  the  form  of  prepotency  ;  and  even  where 
selective  fertility  has  gained  a  footing  in  other  ways, 
the  chances  against  the  suitable  mating  of  "  physio- 
logical complements  "  must  be  much  greater  than  it 
is  in  the  latter  case.  Hence,  among  the  higher  animals, 
selective  fertility  ought  much  more  frequently  to  be 
found  in  association  with  other  forms  of  homogamy 
than  it  is  among  plants.  And  this  is  exactly  what 
we  find.  Thus  it  seems  to  me  that  this  contrast 
between  the  comparative  absence  and  presence  of 
physical  barriers,  where  allied  species  of  plants  and  of 
higher  animals  are  respectively  concerned,  is  entitled 
to  be  taken  as  a  further  corroboration  of  our  theory. 
For  while  it  displays  exactly  such  a  general  corre- 
lation as  this  theory  would  expect,  the  correlation  is 
one  which  cannot  possibly  be  explained  on  any  other 
theory.  It  is  just  where  physiological  selection  can 
be  seen  to  have  the  best  opportunity  of  acting  (viz. 
in  the  vegetable  kingdom)  that  we  find  the  most 
unequivocal  evidence  of  its  action;  while,  on  the 
other  hand,  it  is  just  where  it  can  be  seen  to  have 
the  least  opportunity  of  asserting  itself  (viz.  among 
the  higher  animals)  that  we  find  it  most  associated 
with,  and  therefore  assisted  by,  other  forms  of  homo- 


Evidences  of  Physiological  Selection.    95 

gamy,  i.  e.  not  only  geographical  isolation,  but  also 
by  sexual  preference  in  pairing,  and  the  several 
other  forms  of  homogamy,  which  Mr.  Gulick  has 
shown  to  arise  in  different  places  as  the  result  of 
intelligence. 

Evidence  from  Special  Cases. 

Hitherto  I  have  been  considering,  from  the  most 
general  point  of  view,  the  most  widespread  facts 
and  broadest  principles  which  serve  to  substantiate 
the  theory  of  physiological  selection.  I  now  pass 
to  the  consideration  of  one  of  those  special  cases  in 
which  the  theory  appears  to  have  been  successfully 
applied. 

Professor  Le  Conte  has  adduced  the  fossil  snails 
of  Steinheim  as  serving  to  corroborate  the  theory  of 
physiological  selection  *. 

The  facts  are  these.  The  snail  population  of  this 
lake  remain  for  a  long  time  uniform  and  unchanged. 
Then  a  small  percentage  of  individuals  suddenly  began 
to  vary  as  regards  the  form  of  their  shells,  and  this  in 
two  or  three  directions  at  the  same  time,  each  affected 
individual,  however,  only  presenting  one  of  the  varia- 
tions. But  after  all  these  variations  had  begun  to 
affect  a  proportionally  large  number  of  individuals, 
some  individuals  occur  in  which  two  or  more  of  the 
variations  are  blended  together,  evidently,  as  Weis- 
mann  says,  by  intercrossing  of  the  varieties  so  blended. 
Later  still,  both  the  separate  varieties  and  their 
blended  progeny  became  more  and  more  numerous, 
and  eventually  a  single  blended  type,  comprising 
in  itself  all  the  initial  varieties,  supplanted  the 

1   /.volution  **dtts  Relations  to  Religious  Thought,  &c.  pp.  236-7. 


96          Darwin,  and  after  Darwin. 

parent  form.  Then  another  long  period  of  stability 
ensued  until  another  eruption  of  new  variations  took 
place ;  and  these  variations,  after  having  affected 
a  greater  and  greater  number  of  individuals,  eventu- 
ally blended  together  by  intercrossing  and  sup- 
planted their  parent  form.  So  the  process  went  on, 
comparatively  short  periods  of  variation  alternating 
with  comparatively  long  periods  of  stability,  the 
variations,  moreover,  always  occurring  suddenly  in 
crops,  then  multiplying,  blending  together,  and  in 
their  finally  blended  type  eventually  supplanting  their 
parent  form. 

Now,  the  remarkable  fact  here  is  that  whenever  the 
variations  arose,  they  only  intercrossed  between  them- 
selves, they  did  not  intercross  with  their  parent  form  ; 
for,  if  they  had,  not  only  could  they  never  have 
survived  (having  been  at  first  so  few  in  number  and 
there  having  been  no  geographical  barriers  in  the 
small  lake),  but  we  should  have  found  evidence  of 
the  fact  in  the  half-bred  progeny.  Moreover,  natural 
selection  can  have  had  nothing  to  do  with  the  process,  • 
because  not  only  are  the  variations  in  the  form  of  the 
shells  of  no  imaginable  use  in  themselves  ;  but  it 
would  be  preposterous  to  suppose  that  at  each  of  these 
"variation  periods"  several  different  variations  should 
always  have  occurred  simultaneously,  all  of  which  were 
of  some  hidden  use,  although  no  one  of  them  ever 
occurred  during  any  of  the  prolonged  periods  of 
stability.  How,  then,  are  we  to  explain  the  fact  that 
the  individuals  composi  g  each  crop  of  varieties,  while 
able  to  breed  among  themselves,  never  crossed  with 
their  parent  form?  These  varieties,  each  time  that 
they  arose,  were  intimately  commingled  with  their 


Evidences  of  Physiological  Selection.    97 

parent  form,  and  would  certainly  have  been  re- 
absorbed  into  it  had  intercrossing  in  that  direction 
been  possible.  With  Professor  Le  Conte,  therefore, 
I  conclude  that  there  is  only  one  conceivable  answer 
to  this  question.  Each  crop  of  varieties  must  have 
been  protected  from  intercrossing  with  their  parent 
form. 

They  must  have  been  the  result  of  a  variation,  which 
rendered  the  affected  individuals  sterile  with  their 
parent  form,  whilst  leaving  them  fertile  amongst  them- 
selves. The  progeny  of  these  individuals  would  then 
havedispersed  through  the  lake,  physiologically  isolated 
from  the  parent  population,  and  especially  prone  to 
develop  secondary  variations  as  a  direct  result  of  the 
primary  variation.  Thus,  as  we  might  expect,  two  or 
three  variations  arose  simultaneously,  as  expressions 
of  so  many  different  lines  of  family  descent  from  the 
original  or  physiological  variety  ;  these  were  every- 
where prevented  from  intercrossing  with  their  parent 
form,  yet  capable  of  blending  whenever  they  or  their 
ever-increasing  progeny  happened  to  meet.  Thus, 
without  going  into  further  details,  we  are  able  by 
the  theory  of  physiological  selection  to  give  an  ex- 
planation of  all  these  facts,  which  otherwise  remain 
inexplicable. 

In  view  of  the  evidence  which  has  now  been  pre- 
sented, I  will  now  ask  five  questions  which  must  be 
suitably  answered  by  critics  of  the  theory  of  physio- 
logical selection. 

i.  Can  you  doubt  that  the  hitherto  insoluble  pro- 
blem of  inter-specific  sterility  would  be  solved,  sup- 
posing cross-infertility  were  proved  to  arise  before  or 

III  ,H 


98          Darwin,  and  after  Darwin. 

during  the  process  of  specific  differentiation,  instead 
of  after  that  process  had  been  fully  completed  ? 

2.  Can  you  doubt,  after  duly  considering  the  cir- 
cumstances under  which  allied  species  of  plants  have 
been    differentiated — viz.  in    ninety-five   per  cent,  of 
cases  intimately  commingled  on  common  areas,  and 
therefore  under  identical    environments— that  cross- 
infertility   must   have    arisen   before   or   during    the 
specific  differentiation  ? 

3.  Can  you  doubt,  after  duly  considering  the  facts 
of  prepotency  on  the  one  hand  and  those  of  Jordan's 
physiological  varieties  on  the  other,  that  cross-infer- 
tility does  arise  before  or  during  the  specific  differen- 
tiation ? 

4.  If  you  cannot  express  a  doubt  upon  any  of  these 
points,  can  you  explain  why  you  refuse  to  accept  the 
theory  of  the  origin  of  species  by  means  of  physio- 
logical selection,  together  with  the  explanation  which 
this  theory  affords  of  the  continued  cross-fertility  of 
domesticated  varieties  ? 

5.  Supposing  this  theory  to  be  true,  can  you  con- 
ceive   of   any   other   classes   of   facts   which,   either 
quantitatively  or  qualitatively,  could  more  directly  or 
more  effectually  prove  its  truth  than  those  which  have 
now  been  adduced  ? 

On  these  five  heads  I  entertain  no  doubt.  I  am 
convinced  that  the  theory  of  physiological  selection  is 
the  only  one  that  can  explain  the  facts  of  inter-specific 
sterility  on  the  one  hand,  and,  on  the  other  hand,  the 
contrast  which  these  facts  display  to  the  unimpaired 
fertility  of  our  domesticated  varieties. 

In  conclusion,  it  seems  desirable  once  more  to  insist 
that  there  is  no  antagonism  or  rivalry  between  the 


Evidences  of  Physiological  Selection.    99 

theories  of  natural  and  of  physiological  selection.  For 
which  purpose  I  will  quote  the  final  paragraph  of  my 
original  paper. 

So  much,  then,  for  the  resemblances  and  the  differences 
between  the  two  theories.  It  only  remains  to  add  that  the  two 
are  complementary.  I  have  already  shown  some  of  the  respects 
in  which  the  newer  theory  comes  to  the  assistance  of  the  older, 
and  this  in  the  places  where  the  older  has  stood  most  in  need  of 
assistance.  In  particular,  I  have  shown  that  segregation  of  the 
fit  entirely  relieves  survival  of  the  fittest  from  the  difficulty  under 
which  it  has  hitherto  laboured  of  explaining  why  it  is  that  sterility 
is  so  constantly  found  between  species,  while  so  rarely  found 
between  varieties  which  differ  from  one  another  even  more  than 
many  species  ;  why  so  many  features  of  specific  distinction  are 
useless  to  the  species  presenting  them  ;  and  why  it  is  that 
incipient  varieties  are  not  obliterated  by  intercrossing  with  parent 
forms.  Again,  we  have  seen  that  physiological  selection,  by 
preventing  such  intercrossing,  enables  natural  selection  to 
promote  diversity  of  character,  and  thus  to  evolve  species  in 
ramifying  branches  instead  of  in  linear  series— a  work  which  I 
cannot  see  how  natural  selection  could  possibly  perform  unless 
thus  aided  by  physiological  selection.  Moreover,  we  have  seen 
that  although  natural  selection  alone  could  not  induce  sterility 
between  allied  types,  yet  when  this  sterility  is  given  by  physio- 
logical selection,  the  forms  which  present  it  would  be  favoured  in 
the  struggle  for  existence  ;  and  thus  again  the  two  principles  are 
found  playing,  as  it  were,  into  each  other's  hands.  And  here,  as 
elsewhere,  I  believe  that  the  co-operation  enables  the  two  prin- 
ciples to  effect  very  much  more  in  the  way  of  species-making 
than  either  of  them  could  effect  if  working  separately.  On  the 
one  hand,  without  the  assistance  of  physiological  selection, 
natural  selection  would,  I  believe,  be  all  but  overcome  by  the 
adverse  influences  of  free  intercrossing — influences  all  the  more 
potent  under  the  very  conditions  which  are  required  for  the 
multiplication  of  species  by  divergence  of  character.  On  the 
other  hand,  without  natural  selection,  physiological  selection 
would  be  powerless  to  create  any  differences  of  specific  type, 
other  than  those  of  mutual  sterility  and  trivial  details  of  structure, 
II  2 


ioo         Darwin,  and  after  Darwin. 

form,  and  colour -differences  wholly  without  meaning  from  a 
utilitarian  point  of  view.  But  in  their  combination  these  two 
principles  appear  to  me  able  to  accomplish  what  neither  can 
accomplish  alone— namely,  a  full  and  satisfactory  explanation  of 
the  origin  of  species. 


CHAPTER  VI. 

A  BRIEF  HISTORY  OF  OPINIONS  ON  ISOLATION 
AS  A  FACTOR  OF  ORGANIC  EVOLUTION. 

THIS  historical  sketch  must  begin  with  a  considera- 
tion of  Darwin's  opinions  on  the  subject ;  but  as  these 
were  considerably  modified  from  time  to  time  during 
a  period  of  thirty  years  by  the  publications  of  other 
naturalists,  it  will  be  impossible  to  avoid  cross- 
references  as  between  his  writings  and  theirs.  It 
may  also  be  observed  that  the  Life  and  Letters  of 
Charles  Darwin  was  not  published  until  the  year 
1887,  so  that  the  various  opinions  which  I  shall 
quote  from  the  letters,  and  which  show  some  con- 
siderable approximation  in  his  later  years  to  the 
views  which  have  been  put  forward  by  Mr.  Gulick 
and  myself,  were  not  before  us  at  the  time  when  our 
papers  were  read. 

The  earliest  allusion  that  I  can  find  to  geographical 
isolation  in  the  writings  of  Darwin  occurs  in  a 
correspondence  with  Sir  Joseph  Hooker,  as  far  back 
as  1844.  He  there  says  : — 

I  cannot  give  my  reasons  in  detail ;  but  the  most  general 
conclusion  which  the  geographical  distribution  of  all  organic 


102         Darwin,  and  after  Darwin. 

beings  appears  to  me  to  indicate  is,  that  isolation  is  the  chief 
concomitant  or  cause  of  the  appearance  of  new  forms  (I  well 
know  there  are  some  staring  exceptions) '. 

And  again : — 

With  respect  to  original  creation  or  production  of  new  forms, 
I  have  said  that  isolation  appears  the  chief  element 2. 

Next,  in  the  earlier  editions  of  the  Origin  of  Species 
this  view  is  abandoned,  and  in  its  stead  we  meet 
with  the  opinion  that  geographical  isolation  lends 
a  certain  amount  of  assistance  to  natural  selection, 
by  preventing  free  intercrossing.  But  here  we  must 
note  two  things.  First,  the  distinction  between  mono- 
typic  and  polytypic  evolution  is  not  defined.  Secondly, 
the  levelling  effect  of  free  intercrossing  in  nature,  and 
hence  its  antagonism  to  divergence  of  character  by 
natural  selection,  is  not  sufficiently  recognized  ;  while, 
on  the  other  hand,  and  in  consequence  of  this,  the 
importance  of  isolation  as  a  factor  of  evolution  is 
underrated — not  only  in  its  geographical,  but  likewise 
in  all  its  other  forms. 

Taking  these  two  points  separately,  the  only 
passages  in  Darwin's  writings,  so  far  at  least  as  I 
can  find,  in  which  any  distinction  is  drawn  between 
evolution  as  monotypic  and  polytypic,  are  those  in 
which  he  deals  with  a  somewhat  analogous  distinction 
between  artificial  selection  as  intentional  and  un- 
conscious. He  says,  for  example: — 

In  the  case  of  methodical  selection,  a  breeder  selects  for  some 
definite  object,  and  if  the  individuals  be  allowed  freely  to  inter- 
cross, his  work  will  completely  fail.  But  when  many  men, 
without  intending  to  alter  the  breed,  have  a  nearly  common 

1  Life  and  Letters,  vol.  ii.  p.  28.  »  Ibid. 


Opinions  on  Isolation.  103 

standard  of  perfection,  and  all  try  to  procure  and  breed  from  the 
best  animals,  improvement  surely  but  slowly  follows  from  this 
unconscious  process  of  selection,  notwithstanding  that  there  is  no 
separation  of  selected  individuals.  Thus  it  will  be  under  nature  l. 

Here  we  have  what  may  perhaps  be  regarded  as  a 
glimmering  of  the  distinction  between  monotypic  and 
polytypic  evolution.  But  that  it  is  only  a  glimmering 
is  proved  by  the  immediately  ensuing  sentences,  which 
apply  this  analogy  of  unconscious  selection,  not  to  the 
case  of  monotypic,  but  to  that  of  polytypic  evolution. 
So  likewise,  in  the  succeeding  discussion  on  "divergence 
of  character,"  the  analogy  is  again  resorted  to  for  the 
purpose  of  showing  how  polytypic  evolution  may  occur 
in  nature. 

Thus  far,  then,  it  may  be  said  that  we  have  scarcely 
so  much  as  a  glimmering  of  the  distinction  between 
monotypic  and  polytypic  evolution  ;  and  as  the  same 
discussion  (with  but  a  few  verbal  alterations)  runs 
through  all  the  editions  of  the  Origin,  it  may  well  be 
asked  why  I  should  have  alluded  to  such  passages  in 
the  present  connexion.  Well,  I  have  done  so  because 
it  is  apparent  that,  during  the  last  years  of  his  life,  the 
distinction  between  selection  as  "methodical"  and 
"  unconscious  "  enabled  Darwin  much  more  clearly  to 
perceive  that  between  evolution  as  monotypic  and 
polytypic.  Thus  in  1868  he  wrote  to  Moritz  Wagner 
(who,  as  we  shall  presently  see,  entirely  failed  to 
distinguish  between  monotypic  and  polytypic  evolu- 
tion), expressing  his  belief — 

That  in  many  large  areas  all  the  individuals  of  the  same 
species  have  been  slowly  modified,  in  the  same  manner,  for 
instance,  as  the  English  racehorse  has  been  improved,  that  is, 

1  Origin  of  Species,  p.  80,  6th  ed.  (1872). 


104        Darwin,  and  after  Darwin. 

by  the  continued  selection  of  the  fleetest  individuals,  without 
any  separation.  But  I  admit  that  by  this  process  two  or 
more  new  species  could  hardly  be  formed  within  the  same  limited 


Again,  in  1876  he  wrote  another  letter  to  Wagner, 
in  which  the  following  passage  occurs  : — 

I  believe  that  all  the  individuals  of  a  species  can  be  slowly 
modified  within  the  same  district,  in  nearly  the  same  manner  as 
man  effects  by  what  I  have  called  the  process  of  unconscious 
selection.  I  do  not  believe  that  one  species  will  give  birth  to 
two  or  more  new  species  as  long  as  they  are  mingled  together 
within  the  same  district*. 

Two  years  later  he  wrote  to  Professor  Semper : — 

There  are  two  different  classes  of  cases,  it  appears  to  me, 
viz.  those  in  which  species  becomes  slowly  modified  in  the 
same  country,  and  those  cases  in  which  a  species  splits  into  two, 
or  three,  or  more  new  species  ;  and,  in  the  latter  case,  I  should 
think  nearly  perfect  separation  would  greatly  aid  in  their 
"specification,"  to  coin  a  new  word8. 

Now,  these  passages  show  a  very  much  clearer 
perception  of  the  all-important  distinction  between 
monotypic  and  polytypic  evolution  than  any  which 
occur  in  the  Origin  of  Species ;  and  they  likewise 
show  that  he  was  led  to  this  perception  through  what 
he  supposed  to  be  a  somewhat  analogous  distinction 
between  "  unconscious  "  and  "  methodical "  selection 
by  man.  The  analogy,  I  need  hardly  say,  is  radically 
unsound  ;  and  it  is  a  curious  result  of  its  unsoundness 
that,  whereas  in  the  Origin  of  Species  it  is  adduced 
to  illustrate  the  process  of  polytypic  evolution,  as 
previously  remarked,  in  the  letters  above  quoted  we 

1  Life  and  Letters,  vol.  iii.  p.  158. 
•MM.  p.  159.  3  Ibid.  p.  160. 


Opinions  on  Isolation.  105 

find  it  adduced  to  illustrate  the  process  of  monotypic 
evolution.  But  the  fact  of  this  analogy  being  unsound 
does  not  affect  the  validity  of  the  distinction  between 
monotypic  and  polytypic  evolution  to  which  it  led 
Darwin,  in  his  later  years,  so  clearly  to  express  l. 

Turning  next  to  the  second  point  which  we  have  to 
notice,  it  is  easy  to  show  that  in  the  earlier  editions 
of  his  works  Darwin  did  not  sufficiently  recognize 
the  levelling  effects  of  free  intercrossing,  and  conse- 
quently failed  to  perceive  the  importance  of  isolation 
(in  any  of  its  forms)  as  a  factor  of  organic  evolution. 
This  may  be  most  briefly  shown  by  quoting  his  own 
more  matured  opinion  upon  the  subject.  Thus,  with 
reference  to  the  swamping  effects  of  intercrossing,  he 
wrote  to  Mr.  Wallace  in  1 867  as  follows : — 

I  must  have  expressed  myself  atrociously :  I  meant  to  say 
exactly  the  reverse  of  what  you  have  understood.  F.  Jenkin 
argued  in  the  North  British  Review  against  single  variations 
being  perpetuated,  and  has  convinced  me,  though  not  in  quite 
so  broad  a  manner  as  here  put.  I  always  thought  individual 
differences  more  important ;  but  I  was  blind,  and  thought  that 
single  variations  might  be  preserved  much  oftener  than  I  now 
see  is  possible  or  probable.  I  mentioned  this  in  my  former 

1  The  analogy  is  radically  unsound  because  unconscious  selection 
differs  from  methodical  selection  only  in  the  degree  of  "separation" 
which  it  effects.  These  two  forms  of  selection  do  not  necessarily  differ 
from  one  another  in  regard  to  the  number  of  characters  which  are  being 
simultaneously  diversified  ;  for  while  it  may  be  the  object  of  methodical 
selection  to  breed  for  modification  of  a  single  character  alone,  it  may, 
on  the  other  hand,  be  the  result  of  unconscious  selection  to  diversify  an 
originally  uniform  stock,  as  Darwin  himself  observes  with  regard  to 
horse-breeding.  The  real  distinction  between  monotypic  and  polytypic 
evolution  is,  not  at  all  with  reference  to  the  degree  of  isolation  (i.  e. 
amount  of  "  separation  "),  but  to  the  number  of  cases  in  which  any 
efficient  degree  of  it  occurs  (i.  e.  whether  in  but  a  single  case,  or  in  two 
or  more  cases). 


io6         Darivin,  and  after  Darwin. 

note  merely  because  I  believed  that  you  hnd  come  to  a  similar 
conclusion,  and  I  like  much  to  be  in  accord  with  you.  I  believe 
I  was  mainly  deceived  by  single  variations  offering  such  simple 
illustrations,  as  when  man  selects  [i.e.  isolates]1. 

Again,  somewhere  about  the  same  time,  he  wrote 
to  Moritz  Wagner  : — 

Although  I  saw  the  effects  of  isolation  in  the  case  of  islands 
and  mountain-ranges,  and  knew  of  a  few  instances  of  rivers, 
yet  the  greater  number  of  your  facts  were  quite  unknown  to  me. 
I  now  see  that,  from  the  want  of  knowledge,  I  did  not  make 
nearly  sufficient  use  of  the  views  which  you  advocate8. 

Now  it  would  be  easy  to  show  the  justice  of  these 
self-criticisms  by  quoting  longer  passages  from  earlier 
editions  of  the  Origin  of  Species ;  but  as  this,  in  view 
of  the  above  passages,  is  unnecessary,  we  may  next 
pass  on  to  another  point. 

The  greatest  oversight  that  Wagner  made  in  his 
otherwise  valuable  essays  on  geographical  isolation, 
was  in  not  perceiving  that  geographical  isolation  is  only 
one  among  a  number  of  other  forms  of  isolation : 
and,  therefore,  that  although  it  is  perfectly  true,  as 
he  insisted,  that  polytypic  evolution  cannot  be  effected 
by  natural  selection  alone,  it  is  very  far  from  true, 
as  he  further  insisted,  that  geographical  isolation  is 
the  only  means  whereby  natural  selection  can  be 
assisted  in  this  matter.  Hence  it  is  that,  when 
Darwin  said  he  had  not  himself  "  made  nearly 
sufficient  use"  of  geographical  isolation  as  a  factor 
of  specific  divergence,  he  quite  reasonably  added  that 
he  could  not  go  so  far  as  Wagner  did  in  regarding 
such  isolation  as  a  condition,  sine  qiia  non,  to  diver- 
gent evolution  in  all  cases.  Nevertheless,  he  adds 

1  Life  and  Letters,  vol.  iii.  pp.  157-8.  *  Ibid.  pp.  157-8. 


Opinions  on  Isolation.  107 

the  important  words,  '•  I  almost  wish  I  could  believe 
in  its  importance  to  the  same  extent  with  you  ;  for 
you  well  show,  in  a  manner  which  never  occurred  to 
me,  that  it  removes  many  difficulties  and  objections." 
These  words  are  important,  because  they  show  that 
Darwin  had  come  to  feel  the  force  of  the  "  difficulties 
and  objections"  with  regard  to  divergent  evolution 
being  possible  by  means  of  natural  selection  alone, 
and  how  readily  they  could  be  removed  by  assuming 
the  assistance  of  isolation.  Hence,  it  is  much  to  be 
deplored  that  Wagner  presented  a  single  kind  of 
isolation  (geographical)  as  equivalent  to  the  principle 
of  isolation  in  general.  For  he  thus  failed  to  present 
the  complete — and,  therefore,  the  true — philosophy 
of  the  subject  to  Darwin's  mind  ;  and  in  this,  as 
in  certain  other  respects  which  I  shall  notice  later 
on,  served  rather  to  confuse  than  to  elucidate  the 
matter  as  a  whole. 

To  sum  up.  Although  in  his  later  years,  as  shown 
by  his  correspondence,  Darwin  came  to  recognize 
more  fully  the  swamping  effects  of  free  intercrossing, 
and  the  consequent  importance  of  "  separation "  for 
the  prevention  of  these  effects,  and  although  in  this 
connexion  he  likewise  came  more  clearly  to  dis- 
tinguish between  the  "  two  cases "  of  monotypic 
and  polytypic  evolution,  it  is  evident  that  he  never 
worked  out  any  of  these  matters — "  thinking  it  pru- 
dent," as  he  wrote  with  reference  to  them  in  1878, 
'•  now  I  am  growing  old,  to  work  at  easier  subjects1." 
Therefore  he  never  clearly  saw,  on  the  one  hand, 
that  free  intercrossing,  far  from  constituting  a  "  diffi- 
culty ''  to  monotypic  evolution  by  natural  selection, 
1  Life  and  Letters,  vol.  iii.  p.  161. 


io8         Darwin,  and  after  Darwin. 

is  the  very  means  whereby  natural  selection  is  in 
this  case  enabled  to  operate;  or,  on  the  other 
hand,  that,  in  the  case  of  polytypic  evolution,  the 
"  difficulty  "  in  question  is  so  absolute  as  to  render 
such  evolution,  by  natural  selection  alone,  absolutely 
impossible.  Hence,  although  in  one  sentence  of  the 
Origin  of  Species  he  mentions  three  forms  of  isolation 
(besides  the  geographical  form)  as  serving  in  some 
cases  to  assist  natural  selection  in  causing  "diver- 
gence of  character "  (i.  e.  polytypic  evolution J),  on 
account  of  not  perceiving  how  great  and  how  sharp 
is  the  distinction  between  the  two  kinds  or  "  cases  " 
of  evolution,  he  never  realized  that,  where  "  two  or 
more  new  species"  are  in  course  of  differentiation. 
some  form  of  isolation  other  than  natural  selection 
must  necessarily  be  present,  whether  or  not  natural 
selection  be  likewise  so.  The  nearest  approach  which 
he  ever  made  to  perceiving  this  necessity  was  in  one 
of  his  letters  to  Wagner  above  quoted,  where,  after 
again  appealing  to  the  erroneous  analogy  between 
monotypic  evolution  and  "  unconscious  selection,"  he 
says: — "But  I  admit  that  by  this  process  (i.e.  un- 
conscious selection)  two  or  more  new  species  could 
hardly  be  formed  within  the  same  limited  area:  some 
degree  of  separation,  if  not  indispensable,  would  be 
highly  advantageous  ;  and  here  your  facts  and  views 
will  be  of  great  value."  But  even  in  this  passage  the 
context  shows  that  by  "  separation "  he  is  thinking 
exclusively  of  geographical  separation,  which  he  rightly 
enough  concludes  (as  against  Wagner)  need  certainly 

1  Page  81.  The  three  forms  of  isolation  mentioned  are,  "from 
haunting  different  stations,  from  breeding  at  slightly  different  seasons,  or 
from  the  individuals  of  each  variety  preferring  to  pair  together." 


Opinions  on  Isolation.  109 

not  be  "indispensable."  Had  he  gone  a  step  further, 
he  must  have  seen  that  separation,  in  some  form 
or  another,  is  "  indispensable  "  to  polytypic  evolution. 
Instead  of  taking  this  further  step,  however,  two  years 
later  he  wrote  to  Semper  as  follows : — 

I  went  as  far  as  I  could,  perhaps  too  far,  in  agreement  with 
Wagner  [i.  e.  in  the  last  edition  of  the  Origin  of  Species} ;  since 
that  time  I  have  seen  no  reason  to  change  my  mind  ;  but  then 
I  must  add  that  my  attention  has  been  absorbed  on  other 
subjects J. 

And  he  seems  to  have  ended  by  still  failing  to 
perceive  that  the  explanation  which  he  gives  of 
"  divergence  of  character  "  in  the  Origin  of  Species, 
can  only  hold  on  the  unexpressed  assumption  that 
free  intercrossing  is  in  some  way  prevented  at  the 
commencement  and  throughout  the  development,  of 
each  diverging  type. 

Lastly,  we  have  to  consider  Darwin's  opinion  touching 
the  important  principle  of  "  Independent  Variability." 
This,  it  will  be  remembered,  is  the  principle  which 
ensures  that  when  a  portion  (not  too  large)  of  a 
species  is  prevented  from  interbreeding  with  the  rest 
of  the  species,  sooner  or  later  a  divergence  of  type 
will  result,  owing  to  the  fact  that  the  average  qualities 
of  the  separated  portion  at  the  time  of  its  separation 
cannot  have  been  exactly  the  same  as  the  average 
qualities  of  the  specific  type  as  a  whole.  Thus  the 
state  of  Amixia,  being  a  state  of  what  Mr.  Gulick 
calls  Independent  Generation,  will  of  itself— i.  e.  even 
if  unassisted  by  natural  selection— induce  divergence 
of  type,  in  a  ratio  that  has  been  mathematically 
calculated  by  Delboeuf. 

1  Life  and  Letters,  voL  iii.  p.  159. 


no        Darwin,  and  after  Darwin. 

Darwin  wrote  thus  to  Professor  Weismann  in 
1873:— 

I  have  now  read  your  essay  with  very  great  interest.  Your 
view  of  the  origin  of  local  races  through  "  Amixia  "  is  altogether 
new  to  me,  and  seems  to  throw  an  important  light  on  an  obscure 
question 1. 

And  in  the  last  edition  of  the  Variation  of  Animals 
and  Plants  he  adds  the  following  paragraph  : — 

This  view  may  throw  some  light  on  the  fact  that  the  domestic 
animals  which  formerly  inhabited  the  several  districts  in  Great 
Britain,  and  the  half-wild  cattle  lately  kept  in  several  British 
parks,  differed  slightly  from  one  another;  for  these  animals  were 
prevented  from  wandering  over  the  whole  country  and  inter- 
crossing, but  would  have  crossed  freely  within  each  district  or 
park2. 

Now,  although  I  allow  that  Darwin  never  attri- 
buted to  this  principle  of  Amixia.  or  Independent 
Variability,  anything  like  the  degree  of  importance 
to  which,  in  the  opinion  of  Delbceuf,  Gulick,  Giard, 
and  myself,  it  is  entitled,  the  above  passage  appears 
to  show  that,  as  soon  as  the  "view"  was  clearly 
'  suggested  "  to  his  mind,  he  was  so  far  from  being 
unfavourably  disposed  towards  it,  that  he  added 
a  paragraph  to  the  last  edition  of  his  Variation  for 
the  express  purpose  of  countenancing  it.  Never- 
theless, later  on  the  matter  appears  to  have  entirely 
escaped  his  memory  ;  for  in  1878  he  wrote  to  Semper, 
that  he  did  "  not  see  at  all  more  clearly  than  I  did 
before,  from  the  numerous  cases  which  he  [Wagner] 
has  brought  forward,  how  and  why  it  is  that  a  long 
isolated  form  should  almost  always  become  slightly 
modified  s."  I  think  this  shows  entire  forgetfulness 

1  Life  and  Letters,  vol.  iii.  p.  155.         "  I'ariation,  &c.,  vol.  ii.  p.  362. 
*  Lift  and  Letters,  vol.  iii.  p.  161. 


Opinions  on  Isolation.  in 

of  the  principle  in  question,  because,  if  the  latter  is 
good  for  explaining  the  initial  divergence  of  type  as 
between  separated  stocks  of  "  domesticated  animals," 
much  more  must  it  be  competent  to  explain  the 
further  divergence  of  type  which  is  "almost  always" 
observable  in  the  case  of  "  a  long  isolated  form " 
under  nature.  The  very  essence  of  the  principle 
being  that,  when  divergence  of  type  has  once  begun, 
this  divergence  must  ipso  facto  proceed  at  an  ever-ac- 
celerating pace,  it  is  manifestly  inconsistent  to  entertain 
the  principle  as  explaining  the  first  commencement  of 
divergence,  and  then  to  ignore  it  as  explaining  the 
further  progress  of  divergence.  Hence,  I  can  only 
conclude  that  Darwin  had  forgotten  this  principle 
altogether  when  he  wrote  his  letter  to  Semper  in  1878 
— owing,  no  doubt,  as  he  says  in  the  sentence  which 
immediately  follows,  to  his  having  "not  attended 
much  of  late  years  to  such  questions." 

So  much,  then,  for  Darwin's  opinions.  Next  in 
order  of  time  we  must  consider  Moritz  Wagner's 
essays  on  what  he  called  the  "  Law  of  Migration J ." 
The  merit  of  these  essays  was,  first,  the  firm  ex- 
pression of  opinion  upon  the  swamping  effects  of  free 
intercrossing :  and,  second,  the  production  of  a  large 
body  of  facts  showing  the  importance  of  geographical 
isolation  in  the  prevention  of  these  effects,  and  in 
the  consequent  differentiation  of  specific  types.  On 
the  other  hand,  the  defect  of  these  essays  was,  first, 
not  distinguishing  between  evolution  as  monotypic 
and  polytypic  ;  and,  second,  not  perceiving  that  geo- 

1  Die  Darwin  sche  Theorie  und  das  Migrationsgesct*  (1868)  :  Ueber 
den  l-'.influss  der  geographisc hen  Isolirung,  &c.  (1870). 


ii2         Darwin,  and  after  Darwin. 

graphical  isolation  is  only  one  among  a  number  of 
other  forms  of  isolation.  From  these  two  radical 
oversights — which,  however,  were  shared  by  all  other 
writers  of  the  time,  with  the  partial  exception  of 
Darwin  himself,  as  previously  shown — there  arose  the 
following  and  most  lamentable  errors. 

Over  and  over  again  Moritz  Wagner  insists,  as  con- 
stituting the  fundamental  doctrine  of  his  attempted 
reform  of  Darwinism,  that  evolution  by  natural 
selection  is  impossible,  unless  natural  selection  be 
assisted  by  geographical  isolation,  in  order  to  prevent 
the  swamping  effects  of  intercrossing  1.  Now,  if  instead 
of  "  evolution "  he  had  said  "  divergence  of  type," 
and  if  instead  of  "  geographical  isolation "  he  had 
said  "prevention  of  intercrossing,"  he  would  have 
enunciated  the  general  doctrine  which  it  has  been  the 
joint  endeavour  of  Mr.  Gulick  and  myself  to  set  forth. 
But  by  not  perceiving  that  "  evolution "  is  of  two 
radically  different  kinds — polytypic  and  monotypic — 
he  entirely  failed  to  perceive  that,  while  for  one  of  its 
kinds  the  prevention  of  intercrossing  is  an  absolute 
necessity,  for  the  other  of  its  kinds  the  permission  of 
intercrossing  is  a  necessity  no  less  absolute.  And, 
again,  in  missing  the  fact  that  geographical  isolation 

1  For  instance,  speaking  of  common,  or  continuous  areas,  he  says  : — 
"In  this  case  a  constant  variety,  or  new  species,  cannot  be  produced, 
because  the  free  crossing  of  a  new  variety  with  the  old  unaltered  stock 
will  always  cause  it  to  revert  to  the  original  type ;  in  other  words,  will 
destroy  the  new  form.  The  formation  of  a  real  variety,  which  Darwin, 
as  we  know,  regards  as  the  commencement  of  a  new  species,  will  only 
succeed  when  a  few  individuals,  having  crossed  the  barrier  of  their 
habitat,  are  able  to  separate  themselves  for  a  long  time  from  the  old 
stock."  And  the  last  sentence,  given  as  a  summary  of  his  whole 
doctrine,  is — "The  geographical  isolation  of  the  form,  a  necessary 
consequence  of  migration,  is  the  cause  of  its  typical  character." 


Opinions  on  Isolation.  113 

is  but  one  of  the  many  ways  whereby  intercrossing 
may  be  prevented,  he  failed  to  perceive  that,  even 
as  regards  the  case  of  polytypic  evolution,  he  greatly 
erred  in  representing  this  one  form  of  isolation  as 
being  universally  a  necessary  condition  to  the  process. 
The  necessary  condition  to  this  process  is,  indeed,  the 
prevention  of  intercrossing  by  some  means  or  another  ; 
but  his  unfortunate  insistence  on  geographical  separa- 
tion as  the  only  possible  means  to  this  end— especially 
when  coupled  with  his  no  less  unfortunate  disregard 
of  monotypic  evolution — caused  him  to  hinder  rather 
than  to  advance  a  generalization  which  he  had  only 
grasped  in  part.  And  this  generalization  is.  as  now 
so  repeatedly  stated,  that  while  the  form  of  isolation 
which  we  know  as  natural  selection  depends  for  its 
action  upon  the  intercrossing  of  all  the  individuals 
which  it  isolates  (i.e.  selects),  when  acting  alone 
it  can  produce  only  monotypic  evolution ;  but  that 
when  it  is  supplemented  by  any  of  the  other 
numerous  forms  of  isolation,  it  is  furnished  with 
the  necessary  condition  to  producing  polytypic 
evolution— and  this  in  as  many  lines  of  divergent 
change  as  there  may  be  cases  of  this  efficient 
separation. 

Nevertheless,  while  we  must  lament  these  short- 
comings on  the  part  of  Wagner,  we  ought  to  re- 
member that  he  rendered  important  services  in  the 
way  of  calling  attention  to  the  swamping  effects  of 
free  intercrossing,  and,  still  more,  in  that  of  showing 
the  high  importance  of  geographical  isolation  as  a 
factor  of  organic  evolution.  Therefore,  although  in  an 
elaborate  criticism  of  his  views  Weismann  was  easily 
able  to  dispose  of  his  generalizations  in  the  imperfect 


ii4        Darwin,  and  after  Darwin. 

form  that  they  presented,  I  do  not  think  it  was  just  in 
Weismann  to  remark,  "  if  Wagner  had  confined  himself 
to  the  statement  that  geographical  isolation  materi- 
ally assists  the  process  of  natural  selection,  and 
thus  also  promotes  the  origination  of  new  species,  he 
would  have  met  with  little  or  no  opposition  ;  but  then, 
of  course,  in  saying  this  much,  he  would  not  have 
been  saying  anything  new."  No  doubt,  as  I  have 
just  shown,  he  ought  thus  (as  well  as  in  other  and 
still  more  important  respects  not  perceived  by  Prof. 
Weismann)  to  have  limited  his  statement ;  but,  had 
he  done  so,  it  does  not  follow  that  he  would  not  have 
been  saying  anything  new.  For,  in  point  of  fact,  in 
as  far  as  he  said  what  was  true,  he  did  say  a  great 
deal  that  was  also  new.  Thus,  most  of  what  he  said 
of  the  principle  of  separation  (apogamy)  was  as  new 
as  it  was  true,  although,  as  we  have  seen,  he  said  it 
to  very  little  purpose  on  account  of  his  identifying 
this  principle  as  a  whole  with  that  of  but  one  of  its 
forms.  Again,  notwithstanding  this  great  error,  or 
oversight,  he  certainly  showed  of  the  particular  form 
in  question — viz.  geographical  isolation — that  it  was 
of  considerably  more  importance  than  had  previously 
been  acknowledged.  And  this  was  so  far  a  valuable 
contribution  to  the  general  theory  of  descent. 

Prof.  Weismann's  essay,  to  which  allusion  has  just 
been  made1,  was,  however,  in  all  respects  a  great 
advance  upon  those  of  Wagner.  It  was  not  only 
more  comprehensive  in  its  view  of  the  whole  subject 
of  geographical  isolation,  but  likewise  much  more 
adequate  in  its  general  treatment  thereof.  Its  prin- 

1  Ueber  den  Einfluss  der  Isolirung  auf  die  Artbildung  (1872). 


Opinions  on  Isolation.  115 

cipal  defects,  in  my  judgement,  were,  first,  the  in- 
ordinately speculative  character  of  some  of  its  parts, 
and,  second,  the  restriction  of  its  analysis  to  but  one 
form  of  isolation— a  defect  which  it  shares  with  the 
essays  of  Wagner,  and  in  quite  as  high  a  degree. 
Furthermore,  although  this  essay  had  the  great  merit 
of  enunciating  the  principle  of  Amixia,  it  did  so  in 
a  very  inefficient  manner.  For  not  only  was  this 
principle  adduced  with  exclusive  reference  to  geo- 
graphical isolation,  but  even  in  regard  to  this  one 
kind  of  isolation  it  was  presented  in  a  highly  in- 
consistent manner,  as  I  will  now  endeavour  to  show. 

Weismann  was  led  to  perceive  the  principle  in 
question  by  the  consideration  that  new  specific  char- 
acters, when  they  first  appear,  do  not  all  appear 
together  in  the  same  individuals:  they  appear  one 
in  one  individual^  another  in  another,  a  third  in  a 
third,  &c. ;  and  it  is  only  in  the  course  of  succes- 
sive generations  that  they  all  become  blended  in 
the  same  individuals  by  free  intercrossing.  Hence,  the 
eventually  emerging  constant  or  specific  type  is  the 
resultant  of  all  the  transitory  or  varietal  types,  when 
these  have  been  fused  together  by  intercrossing. 
From  which  Weismann  deduces  what  he  considers 
a  general  law- -namely,  that  "the  constancy  of  a 
specific  type  does  not  arise  suddenly,  but  gradually ; 
and  it  is  established  by  the  promiscuous  crossing 
of  all  individuals1."  From  which  again  it  follows, 
that  this  constancy  must  cease  so  soon  as  the  condition 
which  maintains  it  ceases— i.  e.  so  soon  as  free  inter- 
crossing is  prevented  by  the  geographical  isolation 
of  a  portion  of  the  species  from  its  parent  stock  ! 

1  Loc.  cit.,  p.  43. 
I    2 


n6         Danvin,  and  after  Darwin. 

Now,  to  begin  with,  this  statement  of  the  principle 
in  question  is  not  a  good  statement  of  it.  There  was 
no  need,  while  stating  the  doctrine  that  separation 
induces  differentiation,  to  found  the  doctrine  on  any 
such  highly  speculative  basis.  In  point  of  fact,  there 
is  no  real  evidence  that  specific  types  do  attain 
their  constancy  in  the  way  supposed  ;  nor,  for  the 
purposes  of  the  doctrine  in  question,  is  it  necessary 
that  there  should  be.  For  this  doctrine  does  not 
need  to  show  how  the  constancy  has  been  attained; 
it  only  has  to  show  that  the  constancy  is  maintained 
by  free  intercrossing,  with  the  result  that  when  free 
intercrossing  is  by  any  means  prevented,  divergence 
of  character  ensues.  In  short,  the  correct  way  of 
stating  the  principle  is  that  which  has  been  adopted 
by  Delbceuf  and  Gulick — namely,  the  average  char- 
acters of  a  separated  portion  of  a  species  are  not 
likely  to  be  the  same  as  those  of  the  whole  species ; 
with  the  result  that  divergence  of  type  will  be  set 
up  in  the  separated  portion  by  intercrossing  within 
that  portion.  Or  the  principle  may  be  presented 
as  I  presented  it  under  the  designation  of  "Inde- 
pendent Variability '' — namely,  "  a  specific  type  may 
be  regarded  as  the  average  mean  of  all  individual 
variations,  any  considerable  departure  from  this 
average  mean  being,  however,  checked  by  inter- 
crossing," with  the  result  that  when  intercrossing 
is  prevented  between  a  portion  of  a  species  and 
the  rest  of  the  species.  ';this  population  is  permitted 
to  develop  an  independent  history  of  its  own,  shielded 
from  intercrossing  with  its  parent  form  V 

Not   only,   however,   is   Weismann's    principle    of 

1  Physiological  Seledion,  pp.  348,  389. 


Opinions  on  Isolation.  117 

"Amixia"  thus  very  differently  stated  from  that 
of  my  "  Independent  Variability "  (apogamy),  or 
Gulick's  "  Independent  Generation  "  ;  but,  apparently 
owing  to  this  difference  of  statement,  the  principle 
itself  is  not  the  same.  In  particular,  while  Weismann 
holds  with  us  that  when  new  characters  arise  in 
virtue  of  the  mere  prevention  of  intercrossing  with 
parent  forms  these  new  characters  will  be  of  non- 
utilitarian  kind  *,  he  appears  to  think  that  divergence 
of  character  under  such  circumstances  is  not  likely  to 
go  on  to  a  specific  value.  Now,  it  is  of  importance 
to  observe  why  he  arrives  at  this  conclusion,  which  is 
not  only  so  different  from  that  of  Delboeuf,  Gulick, 
and  myself,  but  apparently  so  inconsistent  with  his 
own  recognition  of  the  diversifying  effect  of  "Amixia'' 
as  regards  the  formation  of  permanent  varieties.  For, 
as  we  have  already  seen  while  considering  Darwin's 
views  on  this  same  principle  of  "  Amixia,"  it  is  highly 
inconsistent  to  recognize  its  diversifying  effect  up  to 
the  stage  of  constituting  fixed  varieties,  and  then  not 
to  recognize  that,  so  much  divergence  of  character 
having  been  already  secured  by  the  isolation  alone, 
much  more  must  further  divergence  continue,  and 
continue  at  an  ever  accelerating  pace— as  Delboeuf 
and  Gulick  have  so  well  shown.  What,  then,  is  the 
explanation  of  this  apparent  inconsistency  on  Weis- 
mann's  part?  The  explanation  evidently  is  that, 
owing  to  his  erroneous  statement  of  the  principle,  he 
misses  the  real  essence  of  it.  For,  in  the  first  place, 
he  does  not  perceive  that  this  essence  consists  in  an 
initial  difference  of  average  characters  on  the  part  of 
the  isolated  colony  as  compared  with  the  rest  of  their 
1  Loc.  fit.,  p.  54. 


n8        Darwin,  and  after  Darwin. 

species.  On  the  contrary,  he  loses  himself  in  a  maze 
of  speculation  about  all  species  having  had  what  he 
calls  "  variation-periods,"  or  eruptions  of  general  varia- 
bility alternating  with  periods  of  repose — both  being 
as  unaccountable  in  respect  of  their  causation  as  they 
are  hypothetical  in  respect  of  their  occurrence.  From 
these  speculations  he  concludes,  that  isolation  of  a 
portion  of  a  species  will  then  only  lead  to  divergence 
of  character  when  the  isolation  happens  to  coincide 
with  a  <:  variation-period  "  on  the  part  of  the  species 
as  a  whole,  and  that  the  divergence  will  cease  so 
soon  as  the  "variation-period  "  ceases.  Again,  in  the 
second  place,  as  previously  remarked,  equally  with 
Wagner  whom  he  is  criticizing,  he  fails  to  perceive 
that  geographical  isolation  is  not  the  only  kind  of 
isolation,  or  the  only  possible  means  to  the  prevention 
of  free  intercrossing.  And  the  result  of  this  oversight 
is,  that  he  thinks  amixia  can  act  but  comparatively 
seldom  upon  sufficiently  small  populations  to  become 
a  factor  of  much  importance  in  the  differentiation  of 
species.  Lastly,  in  the  third  place,  owing  to  his 
favourite  hypothesis  that  all  species  pass  through 
a  "variation-period,"  he  eventually  concludes  that  the 
total  amount  of  divergence  of  type  producible  by 
isolation  alone  (even  in  a  small  population)  can  never 
be  greater  than  that  between  the  extremes  of  varia- 
tion which  occur  within  the  whole  species  at  the  date 
of  its  partition  (p.  75).  In  other  words,  the  possibility 
of  change  due  to  amixia  alone  is  taken  to  be  limited 
by  the  range  of  deviation  from  the  general  specific 
average,  as  manifested  by  different  individual  varia- 
tions, before  the  species  was  divided.  Thus  the 
doctrine  of  amixia  fails  to  recognize  the  law  of 


Opinions  on  Isolation.  119 

Delboeuf,  or  the  cumulative  nature  of  divergence  of 
type  when  once  such  divergence  begins  in  a  separated 
section.  Therefore,  in  this  all-important— and,  indeed, 
essential — respect,  amixia  differs  entirely  from  the 
principle  which  has  been  severally  stated  by  Delbceuf, 
Gulick,  and  myself. 

Upon  the  whole,  then,  we  must  say  that  although 
Professor  Weismann  was  the  first  to  recognize  the 
diversifying  influence  of  merely  indiscriminate  isolation 
per  se  (apogamy),  he  did  so  only  in  part.  He  failed 
to  distinguish  the  true  essence  of  the  principle,  and  by 
overlaying  it  with  a  mass  of  hypothetical  speculation, 
concealed  even  more  of  it  than  he  revealed. 

The  general  theory  of  Isolation,  as  independently 
worked  out  by  Mr.  Gulick  and  myself,  has  already 
been  so  fully  explained,  that  it  will  here  be  sufficient 
merely  to  enumerate  its  more  distinguishing  features. 
These  are,  first,  drawing  the  sharpest  possible  line 
between  evolution  as  monotypic  and  polytypic; 
second,  showing  that  while  for  the  former  the  peculiar 
kind  of  isolation  which  is  presented  by  natural 
selection  suffices  of  itself  to  transform  a  specific  type, 
in  order  to  work  for  the  latter,  or  to  branch  a  specific 
type,  natural  selection  must  necessarily  be  assisted  by 
some  other  kind  of  isolation  ;  third,  that  even  in  the 
absence  of  natural  selection,  other  kinds  of  isolation 
may  be  sufficient  to  effect  specific  divergence  through 
independent  generation  alone ;  fourth,  that,  neverthe- 
less, natural  selection,  where  present,  will  always 
accelerate  the  process  of  divergence;  fifth,  that 
monotypic  evolution  by  natural  selection  depends 
upon  the  presence  of  intercrossing,  quite  as  much  as 


i2o         Darwin,  and  after  Darwin. 

polytypic  evolution  (whether  with  or  without  natural 
selection)  depends  upon  the  absence  of  it ;  sixth,  that, 
having  regard  to  the  process  of  evolution  throughout 
all  taxonomic  divisions  of  organic  nature,  we  must 
deem  the  physiological  form  of  isolation  as  the  most 
important,  with  the  exception  only  of  natural 
selection. 

The  only  difference  between  Mr.  Gulick's  essays 
and  my  own  is,  that,  on  the  one  hand,  he  has 
analyzed  much  more  fully  than  I  have  the  various 
forms  of  isolation ;  while,  on  the  other  hand,  I  have 
considered  much  more  fully  than  he  has  the  particular 
form  of  physiological  isolation  which  so  frequently 
obtains  between  allied  species.  This  particular  form 
of  physiological  isolation  I  have  called  "  physiological 
selection,"  and  claim  for  it  so  large  a  share  in  the 
differentiation  of  specific  types  as  to  find  in  it  a 
satisfactory  explanation  of  the  contrast  between 
natural  species  and  artificial  varieties  in  respect  of 
cross-infertility. 

Mr.  Wallace,  in  his  Darwinism,  has  done  good 
service  by  enabling  all  other  naturalists  clearly  to 
perceive  how  natural  selection  alone  produces  mono- 
typic  evolution — namely,  through  the  free  intercross- 
ing of  all  individuals  which  have  not  been  eliminated  by 
the  isolating  process  of  natural  selection  itself.  For 
he  very  lucidly  shows  how  the  law  of  averages  must 
always  ensure  that  in  respect  of  any  given  specific 
character,  half  the  individuals  living  at  the  same  time 
and  place  will  present  the  character  above,  and  half 
below  its  mean  in  the  population  as  a  whole.  Con- 
sequently, if  it  should  ever  be  of  advantage  to  a  species 


Opinions  on  Isolation.  121 

that  this  character  should  undergo  either  increase  or 
decrease  of  its  average  size,  form,  colour,  &c.,  there 
will  always  be,  in  each  succeeding  generation,  a  suffi- 
cient number  of  individuals — i.  e.  half  of  the  whole — 
which  present  variations  in  the  required  direction, 
and  which  will  therefore  furnish  natural  selection 
with  abundant  material  for  its  action,  without  the 
need  of  any  other  form  of  isolation.  It  is  to  be 
regretted,  however,  that  while  thus  so  clearly  pre- 
senting the  fact  that  free  intercrossing  is  the  very 
means  whereby  natural  selection  is  enabled  to  effect 
monotypic  evolution,  he  fails  to  perceive  that  such 
intercrossing  must  always  and  necessarily  render  it 
impossible  for  natural  selection  to  effect  polytypic 
evolution.  A  little  thought  might  have  shown  him 
that  the  very  proof  which  he  gives  of  the  necessity 
of  intercrossing  where  the  transmutation  of  species 
is  concerned,  furnishes,  measure  for  measure,  as  good 
a  proof  of  the  necessity  of  its  absence  where  the  multi- 
plication of  species  is  concerned.  In  justice  to  him, 
however,  it  may  be  added,  that  this  distinction  be- 
tween evolution  as  monotypic  and  polytypic  (with 
the  important  consequence  just  mentioned)  still  con- 
tinues to  be  ignored  also  by  other  well-known  evo- 
lutionists of  the  "  ultra-Darwinian  "  school.  Professor 
Meldola,  for  example,  has  more  recently  said  that  in 
his  opinion  the  "difficulty  from  intercrossing  "  has  been 
in  large  part — if  not  altogether — removed  by  Mr. 
Wallace's  proof  that  natural  selection  alone  is  capable 
of  effecting  [monotypic]  evolution  ;  while  he  regards 
the  distinction  between  monotypic  and  polytypic 
evolution  as  mere  "verbiage1." 

1  Nature,  vol.  xliii.  p.  410,  and  vol.  xliv.  p.  ap. 


122         Darwin,  and  after  Darwin. 

It  is  in  relation  to  my  presentment  of  the  im- 
possibility of  natural  selection  alone  causing  poly- 
typic  evolution,  that  Mr.  Wallace  has  been  at  the 
pains  to  show  how  the  permission  of  intercrossing 
(panmixia)  is  necessary  for  natural  selection  in  its 
work  of  causing  monotypic  evolution.  And  not  only 
has  he  thus  failed  to  perceive  that  the  "  difficulty " 
which  intercrossing  raises  against  the  view  of  natural 
selection  being  of  itself  capable  of  causing  polytypic 
evolution  in  no  way  applies  to  the  case  of  monotypic ; 
but  as  regards  this  "  difficulty,"  where  it  does  apply, 
he  says: — 

Professor  G.  J.  Romanes  has  adduced  it  as  one  of  the 
difficulties  which  can  alone  be  overcome  by  his  theory  of  physio- 
logical selection l. 

This,  however,  is  a  misapprehension.  I  have  by 
no  means  represented  that  the  difficulty  in  question 
can  alone  be  overcome  by  this  theory.  What  I  have 
represented  is,  that  it  can  be  overcome  by  any  of  the 
numerous  forms  of  isolation  which  I  named,  and 
of  which  physiological  selection  is  but  one.  And 
although,  where  common  areas  are  concerned,  I  believe 
that  the  physiological  form  of  isolation  is  the  most 
important  form,  this  is  a  very  different  thing  from 
entertaining  the  supposition  which  Mr.  Wallace  here 
assigns  to  me. 

I  may  take  this  opportunity  of  correcting  a  some- 
what similar  misunderstanding  which  has  been  more 
recently  published  by  Professor  W.  A.  Herdman,  of 
Liverpool ;  and  as  the  case  which  he  gives  is  one  of 

1  Darwinism,  p.  143. 


Opinions  on  Isolation.  123 

considerable  interest  in  itself,  I  will  quote  his  remarks 
in  extenso.  In  his  Opening  Address  to  the  Liverpool 
Biological  Society ',  Professor  Herdman  said  : — 

Some  of  you  will  doubtless  remember  that  in  last  year's 
address,  while  discussing  Dr.  Romanes'  theory  of  physiological 
selection,  1  quoted  Professor  Flemming  Jenkin's  imaginary  case 
of  a  white  man  wrecked  upon  an  island  inhabited  by  negroes, 
given  as  an  illustration  of  the  supposed  swamping  effect  by 
free  intercrossing  of  a  marked  variety  with  the  parent  species. 
I  then  went  on  to  say  in  criticism  of  the  result  at  which  Jenkin 
arrived,  viz.  that  the  characteristics  of  the  white  man  would  be 
stamped  out  by  intercrossing  with  the  black  : — 

"Two  influences  have,  1  think,  been  ignored,  viz.  atavism, 
or  reversion  to  ancestral  characters,  and  the  tendency  of  the 
members  of  a  variety  to  breed  with  one  another.  Keeping  to 
the  case  described  above,  I  should  imagine  that  the  numbers  of 
intelligent  young  mulattoes  produced  in  the  second,  third,  fourth, 
and  few  succeeding  generations  would  to  a  large  extent  inter- 
marry, the  result  of  which  would  be  that  a  more  or  less  white 
aristocracy  would  be  formed  on  the  island,  including  the  king 
and  all  the  chief  people,  the  most  intelligent  men  and  the  bravest 
warriors.  Then  atavism  might  produce  every  now  and  then 
a  much  whiter  individual— a  reversal  to  the  characteristics  of 
the  ancestral  European— who,  by  being  highly  thought  of  in 
the  whitish  aristocracy,  would  have  considerable  influence 
on  the  colour  and  other  characteristics  of  the  next  generation. 
Now  such  a  white  aristocracy  would  be  in  precisely  the  same 
circumstances  as  a  fa  Durable  variety  competing  with  its  parent 
species,"  &c. 

You  may  imagine  then  my  pleasure  when,  a  few  months  after 
writing  the  above,  I  accidentally  found,  in  a  letter '  written  by  the 
celebrated  African  traveller  Dr.  David  Livingstone  to  Lord 
Granville,  and  dated  "  Unyanyembe,  July  ist,  1872,"  the  follow- 
ing passage  :— 

"  About  five  generations  ago,  a  white  man  came  to  the  high- 
lands of  Basango,  which  are  in  a  line  east  of  the  watershed. 

1  In  Appendix  to  H.  M.  Stanley's  How  I  found  Livingston*,  and  ed. 
London,  1873,  p.  715. 


124        Darwin,  and  after  Darwin. 

He  had  six  attendants,  who  all  died,  and  eventually  their  head- 
man, called  Charura,  was  elected  chief  by  the  Basango.  In 
the  third  generation  he  had  sixty  able-bodied  spearmen  as  lineal 
descendants.  This  implies  an  equal  number  of  the  other  sex. 
They  are  very  light  in  colour,  and  easily  known,  as  no  one  is 
allowed  to  wear  coral  beads  such  as  Charura  brought  except  the 
royal  family.  A  book  he  brought  was  lost  only  lately.  The 
interest  of  the  case  lies  in  its  connexion  with  Mr.  Darwin's 
celebrated  theory  on  the  '  origin  of  species,'  for  it  shows  that  an 
improved  variety,  as  we  whites  modestly  call  ourselves,  is  not  so 
liable  to  be  swamped  by  numbers  as  some  have  thought." 

Here  we  have  a  perfect  fulfilment  of  what  I  last  year,  in 
ignorance  of  this  observation  of  Livingstone's,  predicted  as  being 
likely  to  occur  in  such  a  case.  We  have  the  whitish  aristocracy 
in  a  dominant  condition,  and  evidently  in  a  fair  way  to  spread 
their  characteristics  over  a  larger  area  and  give  rise  to  a  marked 
variety,  and  it  had  clearly  struck  Livingstone  fourteen  years 
before  the  theory  of  physiological  selection  had  been  heard  of, 
just  as  it  must  strike  us  now,  as  an  instance  telling  strongly 
against  the  "  swamping  "  argument  as  used  by  Flemming  Jenkin 
and  Romanes. 

Here  we  have  a  curious  example  of  one  writer 
supporting  the  statements  of  another,  while  appear- 
ing to  be  under  the  impression  that  he  is  controvert- 
ing those  statements.  Both  Professor  Herdman's 
imaginary  case,  and  its  realization  in  Livingstone's 
account,  go  to  show  "  the  tendency  of  the  members 
of  a  variety  to  breed  with  one  another."  This  is 
what  I  have  called  "psychological  selection,"  and, 
far  from  "  ignoring "  it,  I  have  always  laid  stress 
upon  it  as  an  obviously  important  form  of  isolation 
or  prevention  of  free  intercrossing.  But  it  is  a  form 
of  isolation  which  can  only  occur  in  the  higher  animals, 
and,  therefore,  the  whole  of  Professor  Herdman's 
criticism  is  merely  a  restatement  of  my  own  views 
as  already  published  in  the  paper  which  he  is 


Opinions  on  Isolation.  125 

criticizing.  For  all  that  his  argument  goes  to  prove 
is,  first,  the  necessity  for  some  form  of  isolation  if 
the  overwhelming  effects  of  intercrossing  are  to  be 
obviated  ;  and,  secondly,  the  manifest  consequence 
that  where  the  psychological  form  is  unavailable  (as 
in  many  of  the  lower  animals  and  in  all  plants), 
some  other  form  must  be  present  if  divergent  evolu- 
tion is  taking  place  on  a  common  area. 

Seeing  that  so  much  misunderstanding  has  been 
shown  with  reference  to  my  views  on  "  the  swamp- 
ing effects  of  intercrossing,"  and  seeing  also  that 
this  misunderstanding  extends  quite  as  much  to  Mr. 
Gulick's  views  as  to  my  own,  I  will  here  supply 
brief  extracts  from  both  our  original  papers,  for  the 
double  purpose  of  showing  our  complete  agreement, 
and  of  leaving  it  to  be  judged  whether  we  can 
fairly  be  held  responsible  for  the  misunderstanding 
in  question.  After  having  supplied  these  quotations, 
I  will  conclude  this  historical  sketch  by  considering 
what  Mr.  Wallace  has  said  in  reply  to  the  views 
therein  presented.  I  will  transcribe  but  a  single 
passage  from  our  papers,  beginning  with  my  own. 

Any  theory  of  the  origin  of  species  in  the  way  of  descent  must 
be  prepared  with  an  answer  to  the  question,  Why  have  species 
multiplied?  How  is  it  that,  in  the  course  of  evolution,  species 
have  not  simply  become  transmuted  in  linear  series  instead  of 
ramifying  into  branches  ?  This  question  Mr.  Darwin  seeks  to 
answer  "  from  the  simple  circumstance  that  the  more  diversified 
the  descendants  from  any  one  species  becomes  in  structure, 
constitution,  and  habits,  by  so  much  will  they  be  better  enabled 
to  seize  on  many  and  widely  diversified  places  in  the  economy 
of  nature,  and  so  be  enabled  to  increase  in  numbers."  And  he 
proceeds  to  illustrate  this  principle  by  means  of  a  diagram, 


i26        Darwin,  and  after  Darwin. 

showing  the  hypothetical  divergence  of  character  undergone  by 
the  descendants  of  seven  species.  Thus,  he  attributes  divergence 
of  character  exclusively  to  the  influence  of  natural  selection. 

Now,  this  argument  appears  to  me  unassailable  in  all  save 
one  particular;  but  this  is  a  most  important  particular:  the 
argument  wholly  ignores  the  fact  of  intercrossing  with  parent 
forms.  Granting  to  the  argument  that  intercrossing  with  parent 
forms  is  prohibited,  and  nothing  can  be  more  satisfactory.  The 
argument,  however,  sets  out  with  showing  that  it  is  in  limited 
areas,  or  in  areas  already  overstocked  with  the  specific  form  in 
question,  that  the  advantages  to  be  derived  from  diversification 
will  be  most  pronounced.  It  is  where  they  "jostle  each  other 
most  closely"  that  natural  selection  will  set  a  premium  upon 
any  members  of  the  species  which  may  depart  from  the  common 
type.  Now,  inasmuch  as  this  jostling  or  overcrowding  of 
individuals  is  a  needful  condition  to  the  agency  of  natural 
selection  in  the  way  of  diversifying  character,  must  we  not  feel 
that  the  general  difficulty  from  intercrossing  previously  con- 
sidered is  here  presented  in  a  special  and  aggravated  form  ? 
At  all  events,  I  know  that,  after  having  duly  and  impartially 
considered  the  matter,  to  me  it  does  appear  that  unless  the 
swamping  effects  of  intercrossing  with  the  parent  form  on  an 
overcrowded  area  is  in  some  way  prevented  to  begin  with, 
natural  selection  could  never  have  any  material  supplied  by 
which  to  go  on  with.  Let  it  be  observed  that  I  regard  Mr. 
Darwin's  argument  as  perfectly  sound  where  it  treats  of  the 
divergence  of  species,  and  of  their  further  divergence  into  genera  ; 
for  in  these  cases  the  physiological  barrier  is  known  to  be 
already  present.  But  in  applying  the  argument  to  explain 
the  divergence  of  individuals  into  varieties,  it  seems  to  me  that 
here,  more  than  anywhere  else,  Mr.  Darwin  has  strangely  lost 
sight  of  the  formidable  difficulty  in  question ;  for  in  this 
particular  case  so  formidable  does  the  difficulty  seem  to  me, 
that  I  cannot  believe  that  natural  selection  alone  could  produce 
any  divergence  of  specific  character,  so  long  as  all  the  in- 
dividuals on  an  overcrowded  area  occupy  that  area  together. 
Yet,  if  any  of  them  quit  that  area,  and  so  escape  from  the 
unifying  influence  of  free  intercrossing,  these  individuals  also 
escape  from  the  conditions  which  Mr.  Darwin  names  as  those 


Opinions  on  Isolation.  127 

that  are  needed  by  natural  selection  in  order  to  produce  diver- 
gence. Therefore,  it  appears  to  me  that,  under  the  circum- 
stances supposed,  natural  selection  alone  could  not  produce 
divergence ;  the  most  it  could  do  would  be  to  change  the  whole 
specific  type  in  some  one  direction,  and  thus  induce  trans- 
mutation of  species  in  a  linear  series,  each  succeeding  member 
of  which  might  supplant  its  parent  form.  But  in  order  to 
secure  diversity,  multiplication,  or  ramification  of  species, 
it  appears  to  me  obvious  that  the  primary  condition  required  is 
that  of  preventing  intercrossing  with  parent  forms  at  the  origin 
of  each  branch,  whether  the  prevention  be  from  the  first 
absolute,  or  only  partial. 

Now  for  Mr.  Gulick.  a  portion  of  whose  more 
lengthy  discussion  of  the  subject,  however,  is  all  that 
I  need  quote 

Having  found  that  the  evolution  of  the  fitted  is  secured  through 
the  prevention  of  crossing  between  the  better  fitted  and  the  less 
fitted,  can  we  believe  that  the  evolution  of  a  special  race, 
regularly  transmitting  a  special  kind  of  fitness,  can  be  realized 
without  any  prevention  of  crossing  with  other  races  that  have 
no  power  to  transmit  that  special  kind  of  fitness?  Can  we 
suppose  that  any  advantage,  derived  from  new  powers  that 
prevent  severe  competition  with  kindred,  can  be  permanently 
transmitted  through  succeeding  generations  to  one  small  section 
of  the  species  while  there  is  free  crossing  equally  distributed 
between  all  the  families  of  the  species  ?  Is  it  not  apparent  that 
the  terms  of  this  supposition  are  inconsistent  with  the  funda- 
mental laws  of  heredity  ?  Does  not  inheritance  follow  the  lines 
of  consanguinity ;  and  when  consanguinity  is  widely  diffused, 
can  inheritance  be  closely  limited  ?  When  there  is  free  crossing 
between  the  families  of  one  species,  will  not  any  peculiarity 
that  appears  in  one  family  either  be  neutralized  by  crosses 
with  families  possessing  the  opposite  quality,  or,  being  preserved 
by  natural  selection,  while  the  opposite  quality  is  gradually 
excluded,  will  not  the  new  quality  gradually  extend  to  all  the 
branches  of  the  species  ;  so  that,  in  this  way  or  in  that,  increas- 
ing divergence  of  form  will  be  prevented  ? 


128        Darwin,  and  after  Darwin. 

If  the  advantage  of  freedom  from  competition  in  any  given 
variation  depends  on  the  possession,  in  some  degree,  of  new 
adaptations  to  unappropriated  resources,  there  must  be  some 
cause  that  favours  the  breeding  together  of  those  thus  specially 
endowed,  and  interferes  in  some  degree  with  their  crossing 
with  other  variations,  or,  failing  this,  the  special  advantage  will 
in  succeeding  generations  be  lost.  As  some  degree  of  Inde- 
pendent Generation  is  necessary  for  the  continuance  of  the 
advantage,  it  is  evident  that  the  same  condition  is  necessary 
for  the  accumulation  through  Natural  Selection  of  the  powers 
on  which  the  advantage  depends.  The  advantage  of  divergence 
of  character  cannot  be  retained  by  those  that  fail  to  retain  the 
divergent  character;  and  divergent  character  cannot  be  retained 
by  those  that  are  constantly  crossing  with  other  kinds  ;  and  the 
prevention  of  free  crossing  between  those  that  are  equally 
successful  is  in  no  way  secured  by  Natura  Selection. 

So  much,  then,  as  expressive  of  Mr.  Gulick's 
opinion  upon  this  subject.  To  exactly  the  same 
effect  Professor  Lloyd  Morgan  has  recently  published 
his  judgement  upon  it  thus  : — 

That  perfectly  free  intercrossing,  between  any  or  all  of  the 
individuals  of  a  given  group  of  animals,  is,  so  long  as  the 
characters  of  the  parents  are  blended  in  the  offspring,  fatal  to 
divergence  of  character,  is  undeniable.  Through  the  elimination 
of  less  favourable  variations,  the  swiftness,  strength,  and 
cunning  of  a  race  may  be  gradually  improved.  But  no  form  of 
elimination  can  possibly  differentiate  the  group  into  swift, 
strong,  and  cunning  varieties,  distinct  from  each  other,  so  long 
as  all  three  varieties  freely  interbreed,  and  the  characters  of 
the  parents  blend  in  the  offspring.  Elimination  may  and  does 
give  rise  to  progress  in  any  given  group,  as  a  group ;  it  does 
not  and  cannot  give  rise  to  differentiation  and  divergence,  so 
long  as  interbreeding  with  consequent  interblending  of  characters 
be  freely  permitted.  Whence  it  inevitably  follows,  as  a  matter 
of  simple  logic,  that  where  divergence  has  occurred,  inter- 
crossing and  interbreeding  must  in  some  way  have  been 
lessened  or  prevented.  Thus  a  new  factor  is  introduced,  that 


Opinions  on  Isolation.  129 

of  isolation  or  segregation.  And  there  is  no  questioning  the 
fact  that  it  is  of  great  importance.  Its  importance,  indeed,  can 
only  be  denied  by  denying  the  swamping  effects  of  intercrossing, 
and  such  denial  implies  the  tacit  assumption  that  interbreeding 
and  interblending  are  held  in  check  by  some  form  of  segregation. 
The  isolation  explicitly  denied  is  implicitly  assumed '. 

Similarly,  and  still  more  recently,  Professor 
Le  Conte  writes  : — 

It  is  evident,  then,  as  Romanes  claims,  that  natural  selection 
alone  tends  to  monolypic  evolution.  Isolation  of  some  sort 
seems  necessary  to  polytypic  evolution.  The  tree  of  evolution 
under  the  influence  of  natural  selection  alone  grows  palm-like 
from  its  terminal  bud.  Isolation  was  necessary  to  the  starting 
of  lateral  buds,  and  thus  for  the  profuse  ramification  which  is  its 
most  conspicuous  character*. 

In  order  to  complete  this  historical  review,  it  only 
remains  to  consider  Mr.  Wallace's  utterances  upon  the 
subject. 

It  is  needless  to  say  that  he  stoutly  resists  the 
view  of  Weismann,  Delbceuf,  Gulick,  and  myself,  that 
specific  divergence  can  ever  be  due — or,  as  I  under- 
stand him,  even  so  much  as  assisted — by  this  prin- 
ciple of  indiscriminate  isolation  (apogamy).  It  will  be 
remembered,  however,  that  Mr.  Gulick  has  adduced 
certain  general  principles  and  certain  special  facts 
of  geographical  distribution,  in  order  to  prove  that 
apogamy  eventually  leads  to  divergence  of  character, 
provided  that  the  isolated  section  of  the  species  does 
not  contain  any  very  large  number  of  individuals. 
Now,  Mr.  Wallace,  without  making  any  reference  to 
this  argument  of  Mr.  Gulick.  simply  states  the  reverse 
—namely,  that,  as  a  matter  of  fact,  indiscriminate 

1  Animal  Life  and  Intelligence,  pp.  98,  99  (1890-1891). 
•  Tin  Factors  of  Evolution  (1891). 

ni.  K 


130        Darwin,  and  after  Darwin. 

isolation  is  not  found  to  be  associated  with  diverg- 
ence of  character.  For,  he  says,  "  there  is  an  entire 
absence  of  change,  where,  if  this  were  a  vera  causa, 
we  should  expect  to  find  it1."  But  the  only  case 
which  he  gives  is  that  of  Ireland. 

This,  he  says,  furnishes  "  an  excellent  test  case,  for 
we  know  that  it  [Ireland]  has  been  separated  from 
Britain  since  the  end  of  the  glacial  epoch :  .  .  .  yet 
hardly  one  of  its  mammals,  reptiles,  or  land  molluscs 
has  undergone  the  slightest  change2."  Here,  how- 
ever, Mr.  Wallace  shows  that  he  has  failed  to  under- 
stand "the  views  of  those  who,  like  Mr.  Gulick, 
believe  isolation  itself  to  be  a  cause  of  modification 
of  species  "  ;  for  it  belongs  to  the  very  essence  of  these 
views  that  the  efficiency  of  indiscriminate  isolation  as 
a  "  vera  causa  "  of  organic  evolution  varies  inversely 
with  the  number  of  individuals  (i.  e.  the  size  of  the 
species-section)  exposed  to  its  influence.  Therefore, 
far  from  being  "an  excellent  test  case."  the  case 
of  Ireland  is  unsatisfactory.  If  we  are  in  search  of 
excellent  test  cases,  in  the  sense  intended  by  Mr. 
Wallace,  we  ought  not  to  choose  a  large  island, 
which  from  the  time  of  its  isolation  must  have  con- 
tained large  bulks  of  each  of  the  geographically 
separated  species  concerned :  we  ought  to  choose 
cases  where  as  small  a  number  as  possible  of  the 
representatives  of  each  species  were  in  the  first 
instance  concerned.  And,  when  we  do  this,  the 
answer  yielded  by  any  really  "  excellent  test  case  "  is 
unequivocal. 

No  better  test  case  of  this  kind  has  ever  been 
furnished  than  that  of  Mr.  Gulick's  land-shells. 

1  Darwinism,  p.  151.  -  Ibid. 


Opinions  on  Isolation.  131 

which  Mr.  Wallace  is  specially  considering  in  the 
part  of  his  book  where  the  sentence  above  quoted 
occurs.  How,  then,  does  he  meet  this  case?  He 
meets  it  by  assuming  that  in  all  the  numerous 
adjacent  valleys  of  a  small  island  there  must  be 
as  many  differences  of  environment,  each  of  which 
is  competent  to  induce  slight  varietal  changes  on 
the  part  of  its  occupants  by  way  of  natural  selection, 
although  in  no  one  case  can  the  utility  of  these 
slight  changes  be  surmised.  Now,  against  this  ex- 
planation there  are  three  overwhelming  considerations. 
In  the  first  place,  it  is  purely  gratuitous,  or  offered 
merely  in  order  to  save  the  hypothesis  that  there 
can  be  no  other  cause  of  even  the  most  trivial  change 
in  species  than  that  which  is  furnished  by  natural 
selection.  In  the  second  place,  as  Mr.  Gulick  writes 
to  me  in  a  private  letter,  "  if  the  divergence  of 
Sandwich  Island  land  molluscs  is  wholly  due  to 
exposure  to  different  environments,  as  Mr.  Wallace 
argues  on  pages  147-150,  then  there  must  be  com- 
pletely occult  influences  in  the  environment  that 
vary  progressively  with  each  successive  mile.  This 
is  so  violent  an  assumption  that  it  throws  doubt 
on  any  theory  that  requires  such  support."  In  the 
third  place,  the  assumption  that  the  changes  in 
question  must  have  been  due  to  natural  selection, 
is  wholly  incompatible  with  the  facts  of  isolation 
elsewhere — namely,  in  those  cases  where  (as  in  that 
of  Ireland)  a  large  section  of  species,  instead  of 
a  small  section,  has  been  indiscriminately  isolated. 
Mr.  Wallace,  as  we  have  seen,  inadvertently  alludes 
to  these  "  many  other  cases  of  isolation  "  as  evidence 
against  apogamy  being  per  se  a  cause  of  specific 
K  2 


132        Darwin,  and  after  Darwin. 

change.  But  although,  for  the  reason  above  stated, 
they  are  without  relevancy  in  this  respect,  they 
appear  to  me  fatal  to  the  explanation  which  he  gives 
of  specific  changes  under  apogamy  where  only  small 
sections  of  species  are  concerned.  For  example,  can 
it  be  rationally  maintained  that  there  are  more 
differences  of  environment  between  every  two  of 
the  many  contiguous  valleys  of  a  small  island, 
such  as  Mr.  Gulick  describes,  than  there  are  in 
the  incomparably  larger  area  of  the  whole  of 
Ireland?  But,  if  not,  and  if  natural  selection  is 
able  to  work  such  "  occult "  wonders  in  each  succes- 
sive mile  on  the  Sandwich  Islands,  why  has  it  so 
entirely  lost  this  magic  power  in  the  case  of  Ireland 
— or  in  the  "  many  other  cases  of  isolation "  to 
which  Mr.  Wallace  refers?  On  his  theory  there 
is  no  coherent  answer  to  be  given  to  this  question, 
while  on  our  theory  the  answer  is  given  in  the 
very  terms  of  the  theory  itself.  The  facts  are 
plainly  just  what  the  theory  requires  that  they 
should  be ;  and  therefore,  if  they  were  not  as  they 
are,  the  theory  would  be  deprived  of  that  confirma- 
tion which  it  now  derives  from  them. 

Thus,  in  truth,  though  in  an  opposite  way,  the 
case  of  Ireland  is,  as  Mr.  Wallace  says,  "an  excel- 
lent test  case,"  when  once  the  theory  of  apogamy 
as  a  "  vera  causa "  of  specific  change  is  understood  ; 
and  the  effect  of  applying  the  test  is  fully  to  corro- 
borate this  theory,  while  at  the  same  time  it  as 
fully  negatives  the  other.  For  the  consideration 
whereby  Mr.  Wallace  seeks  to  explain  the  inactivity 
of  natural  selection  in  the  case  of  Ireland  is  not 
" coherent."  What  he  says  is,  "That  changes  have 


Opinions  on  Isolation.  133 

not  occurred  through  natural  selection,  is  perhaps 
due  to  the  less  severe  struggle  for  existence,  owing 
to  the  smaller  number  of  competing  species 1."  But 
even  with  regard  to  molluscs  alone,  there  is  a  greatly 
larger  number  of  species  in  Ireland  than  occurs  in 
any  one  valley  of  the  Sandwich  Islands  ;  while  if  we 
have  regard  to  all  the  other  classes  of  animal  life, 
comparison  entirely  fails. 

Much  more  to  the  point  are  certain  cases  which 
were  adduced  long  ago  by  Weismann  in  his  essay 
previously  considered.  Nevertheless,  although  this 
essay  was  published  as  far  back  as  1872,  and, 
although  it  expressly  deals  with  the  question  of 
divergence  of  character  through  the  mere  prevention  of 
intercrossing  (Amixia),  Mr.  Wallace  nowhere  alludes 
to  these  cases  per  contra,  which  are  so  much  more 
weighty  than  his  own  ':  test  case"  of  Ireland.  Of 
such  are  four  species  of  butterflies,  belonging  to  three 
genera 2,  which  are  identical  in  the  polar  regions  and 
in  the  Alps,  notwithstanding  that  the  sparse  Alpine 
populations  have  been  presumably  separated  from 
their  parent  stocks  since  the  glacial  period ;  or  of 
certain  species  of  fresh  water  crustaceans  (Apus\  the 
representatives  of  which  are  compelled  habitually  to 
form  small  isolated  colonies  in  widely  separated 
ponds,  and  nevertheless  exhibit  no  divergence  of 
character,  although  apogamy  has  probably  lasted  for 
centuries.  These  cases  are  unquestionably  of  a  very 
cogent  nature,  and  appear  of  themselves  to  prove 
that  apogamy  alone  is  not  invariably  capable  of 

1  Let.  dt.,  p.  151. 

Namely,    l.ycaena  donztlii,    L.  phtretu,  Argyiutis  pales,  Ercbia 
man  to. 


134        Darwin,  and  after  Darwin. 

inducing  divergence— at  any  rate,  so  rapidly  as  we 
might  expect.  There  appears,  however,  to  be 
another  factor,  the  presence  or  absence  of  which 
makes  a  great  difference.  This,  as  stated  in  the  text, 
is  the  degree  in  which  a  specific  type  is  stable  or 
unstable — liable  or  not  liable  to  vary.  Thus,  for 
example,  the  Goose  is  what  Darwin  calls  an  "  inflex- 
ible "  type  as  compared  with  most  other  domesticated 
birds.  Therefore,  if  a  lot  of  geese  were  to  be  indis- 
criminately isolated  from  the  rest  of  their  species,  the 
probability  is  that  in  a  given  time  their  descendants 
would  not  have  diverged  from  the  parent  type  to  such 
an  extent  as  would  a  similar  lot  of  ducks  under 
similar  circumstances:  the  more  stable  specific  type 
would  require  a  longer  time  to  change  under  the 
influence  of  apogamy  alone.  Now,  the  butterflies 
and  crustaceans  quoted  by  Weismann  may  be  of  a 
highly  stable  type,  presenting  but  a  small  range 
of  individual  variability ;  and,  if  so,  they  would 
naturally  require  a  long  time  to  exhibit  any  change 
of  type  under  the  influence  of  apogamy  alone.  But, 
be  this  as  it  may,  Weismann  himself  adduces  these 
cases  merely  for  the  sake  of  showing  that  there  are 
cases  which  seem  to  tell  against  the  general  prin- 
ciple of  modification  as  due  to  apogamy  alone — i.e. 
the  general  principle  which,  under  the  name  amixia. 
he  is  engaged  in  defending.  And  the  conclusion 
at  which  he  himself  arrives  is,  that  while  it  would 
be  wrong  to  affirm  that  apogamy  must  in  all 
cases  produce  divergence,  we  are  amply  justified 
in  affirming  that  in  many  cases  it  may  have  done 
so ;  while  there  is  good  evidence  to  prove  that  in 
not  a  few  cases  it  has  done  so,  and  therefore 


Opinions  on  Isolation.  135 

should  be  accepted  as  one  of  the  factors  of  organic 
evolution  *. 

My  view  from  the  very  first  has  been  that  variations 
in  the  way  of  cross-infertility  are  of  frequent  occur- 
rence (how,  indeed,  can  they  be  otherwise,  looking 
to  the  complex  conditions  that  have  to  be  satisfied 
in  every  case  of  full  fertility  ?) ;  and,  therefore, 
however  many  of  such  variations  are  destined  to  die 
out,  whenever  one  arises,  "  under  suitable  conditions," 
"  it  must  inevitably  tend  to  be  preserved  as  a  new 
natural  variety,  or  incipient  species."  Among  the 
higher  animals — which  are  "comparatively  few  in 
number  " — I  think  it  probable  that  some  slight  change 
of  form,  colour,  habit,  &c.,  must  be  usually  needed 
either  to  "  superinduce,"  or,  which  is  quite  a  dif- 
ferent thing,  to  coincide  with  the  physiological  change. 
But  in  the  case  of  plants  and  the  lower  inverte- 
brata,  I  see  no  reason  for  any  frequent  concomitance 
of  this  kind  ;  and  therefore  believe  the  physiological 

1  Since  the  above  was  written,  I  have  heard  of  some  cases  which  seem 
to  present  greater  difficulties  to  our  theory  than  those  above  quoted. 
These  refer  to  some  of  the  numerous  species  of  land  mollusca  which 
inhabit  the  isolated  rocks  near  Madeira  (Dezertas).  My  informant  is 
Dr.  Grabham,  who  has  himself  investigated  the  matter,  and  reports 
as  follows : — 

"  It  is  no  uncommon  thing  to  meet  with  examples  of  the  same  species, 
sub-fossil,  recent,  and  living  upon  one  spot,  and  presenting  no  variation 
in  the  long  record  of  descent"  Then,  after  naming  these  examples,  he 
adds,  "  All  seem  to  vary  immediately  on  attaining  new  ground,  assuming 
many  aspects  in  different  districts." 

Unquestionably  these  statements  support,  in  a  very  absolute  manner, 
Mr.  Wallace's  opinion,  while  making  directly  against  my  own.  It  is 
but  fair,  however,  to  add  that  the  cases  are  not  numerous  vsome  half- 
dozen  at  the  most,  and  all  within  the  limits  of  a  single  genus\  and  that, 
even  in  the  opinion  of  my  informant  himself,  the  facts  have  not  hitherto 
been  sufficiently  investigated  for  any  decisive  judgement  to  be  formed 
upon  them. 


136        Darwin,  and  after  Darwin. 

change  to  be,  "as  a  general  rule,"  the  primordial 
change.  At  the  same  time,  I  have  always  been 
careful  to  insist  that  this  opinion  had  nothing  to  do 
with  "the  essence  of  physiological  selection";  seeing 
that  "it  was  of  no  consequence"  to  the  theory  in 
what  proportional  number  of  cases  the  cross-sterility 
had  begun  per  se,  had  been  superinduced  by  morpho- 
logical changes,  or  only  enabled  to  survive  by 
happening  to  coincide  with  any  other  form  of 
homogamy.  In  short,  ';  the  essence  of  physiological 
selection"  consists  in  all  cases  of  the  diversifying  effect 
of  cross-infertility,  whensoever  and  howsoever  it  may 
happen  in  particular  cases  to  have  been  caused. 

Thus  I  emphatically  reaffirm  that  "from  the  first 
I  have  always  maintained  that  it  makes  no  essen- 
tial difference  to  the  theory  in  what  proportional 
number  of  cases  they  [the  physiological  variations] 
have  arisen  'alone  in  an  otherwise  undifferentiated 
species ' "  ;  therefore,  "  even  if  I  am  wrong  in  sup- 
posing that  physiological  selection  can  ever  act 
alone,  the  principle  of  physiological  selection,  as  I 
have  stated  it,  is  not  thereby  affected.  And  this 
principle  is,  as  Mr.  Wallace  has  re-stated  it,  'that 
some  amount  of  infertility  characterizes  the  distinct 
varieties  which  are  in  process  of  differentiation  into 
species' — infertility  whose  absence,  'to  obviate  the 
effects  of  intercrossing,  may  be  one  of  the  usual 
causes  of  their  failure  to  become  developed  into 
distinct  species."' 

These  last  sentences  are  quoted  from  the  corre- 
spondence in  Nature^,  and  to  them  Mr.  Wallace  replied 
by  saying,  "  if  this  is  not  an  absolute  change  of  front, 
1  Vol.  xliii.  p.  i  a;. 


Opinions  on  Isolation.      •        137 

words  have  no  meaning  ";  that  "  if  this  is  'the  whole 
essence  of  physiological  selection,'  then  physiological 
selection  is  but  a  re-statement  and  amplification  of 
Darwin's  views ";  that  such  a  "  change  of  front "  is 
incompatible,  not  only  with  my  term  "  physiological 
selection,"  but  also  with  my  having  "  acknowledged 
that  Mr.  Catchpool  had  '  very  clearly  put  forward  the 
theory  of  physiological  selection'";  and  much  more 
to  the  same  effect. 

Now,  to  begin  with,  it  is  due  to  Mr.  Catchpool  to 
state  that  his  only  publication  upon  this  subject  is 
much  too  brief  to  justify  Mr.  Wallace's  inference,  that 
he  supposes  variations  in  the  way  of  cross-infertility 
always  to  arise  "  alone  in  an  otherwise  undifferentiated 
species."  What  Mr.  Catchpool's  opinion  on  this 
point  may  be,  I  have  no  knowledge  ;  but,  whatever  it 
is,  he  was  unquestionably  the  first  writer  who  "clearly 
stated  the  leading  principles"  of  physiological  selec- 
tion, and  this  fact  I  am  very  glad  to  have  "  acknow- 
ledged." In  my  correspondence  with  Mr.  Wallace, 
however,  I  not  only  named  Mr.  Catchpool:  I  also 
named— and  much  more  prominently — Mr.  Gulick. 
For  even  if  I  were  to  grant  (which  I  am  far  indeed 
from  doing)  that  there  was  any  want  of  clearness  in 
my  own  paper  touching  the  point  in  question,  I  have 
now  repeatedly  shown  that  it  is  simply  impossible 
for  any  reader  of  Mr.  Gulick's  papers  to  misunder- 
stand his  views  with  regard  to  it.  Accordingly, 
I  replied  to  Mr.  Wallace  in  Nature  by  saying:— 

Not  only  have  I  thus  from  the  first  fully  recognized  the 
sundry  other  causes  of  specific  change  with  which  the  physio- 
logical variations  may  be  associated ;  but  Mr.  Gulick  has  gone 
into  this  side  of  our  common  theory  much  more  fully,  and 


138         Darwin,  and  after  Darwin. 

elaborately  calculated  out  the  high  ratio  in  which  the  differ- 
entiating agency  of  any  of  these  other  causes  must  be  increased 
when  assisted  by— i.  e.  associated  with  —even  a  moderate  degree 
of  the  selective  fertility,  and  vice  versa.  Therefore,  it  is  simply 
impossible  for  Mr.  Wallace  to  show  that  "  our  theory  "  differs 
from  his  in  this  respect.  Yet  it  is  the  only  respect  in  which  his 
reply  alleges  any  difference.  (Vol.  xliii.  p.  127.) 

I  think  it  is  to  be  regretted  that,  in  his  answer  to 
this,  Mr.  Wallace  alludes  only  to  Mr.  Catchpool,  and 
entirely  ignores  Mr.  Gulick — whose  elaborate  calcula- 
tions above  alluded  to  were  communicated  to  the 
Linnaean  Society  by  Mr.  Wallace  himself  in  1887. 

The  time  has  now  come  to  prove,  by  means  of 
quotations,  that"  I  have  from  the  first  represented 
the  "  principle,"  or  "  essence,"  of  physiological  selec- 
tion to  consist  in  selective  fertility  furnishing  a  need- 
ful condition  to  specific  differentiation,  in  at  least 
a  large  proportional  number  of  allied  species  which 
afterwards  present  the  reciprocal  character  of  cross- 
sterility  ;  that  I  have  never  represented  variations 
in  the  way  of  this  selective  fertility  as  necessarily 
constituting  the  initial  variations,  or  as  always  arising 
"alone,  in  an  otherwise  undifferentiated  species"; 
and  that,  although  I  have  uniformly  given  it  as  my 
opinion  that  these  variations  do  in  some  cases  thus 
arise  (especially  among  plants  and  lower  invertebrata), 
I  have  as  uniformly  stated  "that  it  makes  no  differ- 
ence to  the  theory  in  what  proportional  number  of 
cases  they  have  done  so  " — or  even  if,  as  Mr.  Wallace 
supposes,  they  have  never  done  so  in  any  case  at  all l. 

1  This  refers  to  what  I  understand  Mr.  Wallace  to  say  in  the  Nature 
correspondence  is  the  supposition  on  which  his  own  theory  of  the  origin 
of  species  by  cross-infertility  is  founded.  But  in  the  original  statement 
of  that  theory  itself,  it  is  everywhere  "supposed"  that  when  species  are 


Opinions  on  Isolation.  139 

These  statements  (all  of  which  are  contradictory 
of  the  only  points  of  difference  alleged)  have  already 
been  published  in  my  article  in  the  Monist  of 
October,  1890.  And  although  Mr.  Wallace,  in  his 
reply  to  that  article,  ignores  my  references  to  the 
"  original  paper,"  it  is  scarcely  necessary  to  quote  the 
actual  words  of  the  paper  itself,  since  the  reader  who 
is  further  interested  in  this  controversy  can  readily 
refer  to  it  in  the  Journal  of  the  Linnaean  Society 
(vol.  xix.  pp.  337-411). 

Having  arrived  at  these  results  with  regard  to  the 
theory  of  Isolation  in  general  and  of  Physiological 
Isolation  in  particular,  I  arrive  also  at  the  end  of  this 
work.  And  if,  while  dealing  with  the  post- Darwinian 
period,  I  have  imparted  to  any  general  reader  the 
impression  that  there  is  still  a  great  diversity  of 
expert  opinion ;  I  must  ask  him  to  note  that  points 
with  reference  to  which  disagreement  still  exists 
are  but  very  subordinate  to  those  with  regard  to 
which  complete  agreement  now  prevails.  The  noise 
of  wrangling  disputations  which  has  so  filled  the 
camp  of  evolutionists  since  the  death  of  their 
captain,  is  apt  to  hide  from  the  outside  world  the 
solid  unanimity  that  prevails  with  regard  to  all 
the  larger  and  more  fundamental  questions,  which 
were  similarly  the  subjects  of  warfare  in  the  past 
generation.  Indeed,  if  we  take  a  fair  and  general 

originated  by  cross-infertility,  the  initial  change  is  the  physiological 
change.  In  his  original  statement  of  that  theory,  therefore,  he  literally 
went  farther  than  I  had  gone  in  my  "  original  paper,"  with  reference  to 
supposing  the  physiological  change  to  be  the  initial  change.  I  do  not 
doubt  that  this  is  doe  to  some  oversight  of  expression ;  but  it  is  carious 
that,  having  made  it,  he  should  still  continue  hi*  endeavour  to  fir  exactly 
the  same  oversight  upon  me. 


140        Darwin,  and  after  Darwin. 

view  of  the  whole  history  of  Darwinism,  what  must 
strike  us  as  the  really  significant  fact  is  the  astonish- 
ing unanimity  which  has  been  so  rapidly  attained 
with  regard  to  matters  of  such  immeasurable  impor- 
tance. It  is  now  but  little  more  than  thirty  years 
since  the  publication  of  the  Origin  of  Species  \  and 
in  that  period  not  only  have  all  naturalists  unequi- 
vocally embraced  the  doctrine  of  descent  considered 
as  a  fact ;  but,  in  one  degree  or  another,  they  have 
all  as  unequivocally  embraced  the  theory  of  natural 
selection  considered  as  a  method.  The  only  points 
with  regard  to  which  any  difference  of  opinion  still 
exist,  have  reference  to  the  precise  causation  of  that 
mighty  stream  of  events  which,  under  the  name  of 
organic  evolution,  we  have  now  all  learnt  to  accept  as 
scientifically  demonstrated.  But  it  belongs  to  the 
very  nature  of  scientific  demonstration  that,  where 
matters  of  great  intricacy  as  well  as  of  high  generality 
are  concerned,  the  process  of  demonstration  must  be 
gradual,  even  if  it  be  not  always  slow.  It  is  only  by 
the  labours  of  many  minds  working  in  many  directions 
that,  in  such  cases,  truth  admits  of  being  eventually 
displayed.  Line  upon  line,  precept  upon  precept, 
here  a  little  and  there  a  little — such  is  the  course  of 
a  scientific  revelation ;  and  the  larger  the  subject- 
matter,  the  more  subtle  and  the  more  complex  the 
causes,  the  greater  must  be  the  room  for  individual 
differences  in  our  reading  of  the  book  of  Nature. 
Now,  if  all  this  be  true,  must  we  not  feel  that  in  the 
matter  of  organic  evolution  the  measure  of  agreement 
which  has  been  attained  is  out  of  all  proportion  to 
the  differences  which  still  remain — differences  which, 
although  of  importance  in  themselves,  are  insignificant 


Opinions  on  Isolation.  141 

when  compared  with  those  which  once  divided  the 
opinions  of  not  a  few  still  living  men  ?  And  if  we  are 
bound  to  feel  this,  are  we  not  bound  further  to  feel 
that  the  very  intensity  of  our  disputations  over  these 
residual  matters  of  comparative  detail,  is  really  the 
best  earnest  that  can  be  given  of  the  determination 
of  our  quest — determination  which,  like  that  of  our 
fathers,  cannot  fail  to  be  speedily  rewarded  by  the 
discovery  of  truth  ? 

Nevertheless,  so  long  as  this  noise  of  conflict  is 
in  the  Senate,  we  cannot  wonder  if  the  people  are 
perplexed.  Therefore,  in  conclusion.  I  may  ask  it  to 
be  remembered  exactly  what  are  the  questions — and 
the  only  questions — which  still  divide  the  parties. 

Having  unanimously  agreed  that  organic  evolution 
is  a  fact  and  that  natural  selection  is  a  cause,  or 
a  factor  in  the  process,  the  primary  question  in  debate 
is  whether  natural  selection  is  the  only  cause,  or 
whether  it  has  been  assisted  by  the  co-operation  of 
other  causes.  The  school  of  Weismann  maintain  that 
it  is  the  only  cause;  and  therefore  deem  it  worse 
than  useless  to  search  for  further  causes.  With  this 
doctrine  Wallace  in  effect  agrees,  excepting  as  regards 
the  particular  case  of  the  human  mind.  The  school 
of  Darwin,  on  the  other  hand — to  which  I  myself 
claim  to  belong— believe  that  natural  selection  has 
been  to  a  considerable  extent  supplemented  by  other 
factors  ;  and,  therefore,  although  we  further  believe 
that  it  has  been  the  "main"  factor,  we  agree  with 
Darwin  himself  in  strongly  reprobating  all  attempts 
to  bar  a  priori  the  progress  of  scientific  investigation 
touching  what,  if  any,  these  other  factors  may  be. 
Lastly,  there  are  several  more  or  less  struggling 


142         Darwin,  and  after  Darwin. 

schools,  chiefly  composed  of  individual  members  who 
agree  with  each  other  only  to  the  extent  of  holding 
that  the  causal  agency  of  natural  selection  is  not  so 
great  as  Darwin  supposed.  The  Duke  of  Argyll, 
Mr.  Mivart,  and  Mr.  Geddes  may  be  named  in  this 
connexion;  together  with  the  self-styled  neo- 
Lamarckians,  who  seek  to  magnify  the  Lamarckian 
principles  at  the  expense  of  the  distinctively  Dar- 
winian. 

This  primary  difference  of  opinion  leads  deductively 
to  certain  secondary  differences.  For  if  a  man  starts 
with  the  premiss  that  natural  selection  must  neces- 
sarily be  the  "  exclusive "  cause  of  organic  evolution, 
he  is  likely  to  draw  conclusions  which  another  man 
would  not  draw  who  starts  with  the  premiss  that 
natural  selection  is  but  the  "  main  "  cause.  Of  these 
subordinate  differences  the  most  important  are  those 
which  relate  to  the  possible  transmission  of  acquired 
characters,  to  the  necessary  (or  only  general)  utility 
of  specific  characters,  and  to  the  problem  touching  the 
inter-sterility  of  allied  species.  But  we  may  well 
hope  that  before  another  ten  years  shall  have  passed, 
even  these  still  outstanding  questions  will  have  been 
finally  settled  ;  and  thus  that  within  the  limits  of  an 
ordinary  lifetime  the  theory  of  organic  evolution  will 
have  been  founded  and  completed  in  all  its  parts,  to 
stand  for  ever  in  the  world  of  men  as  at  once  the 
greatest  achievement  in  the  history  of  science,  and  the 
most  splendid  monument  of  the  nineteenth  century. 

In  the  later  chapters  of  the  foregoing  treatise  I  have 
sought  to  indicate  certain  matters  of  general  principle, 
which  many  years  of  study  specially  devoted  to  this 
great  movement  of  contemporary  thought  have  led 


Opinions  on  Isolation.  143 

me  to  regard  as  almost  certainly  sound  in  themselves, 
and  no  less  certainly  requisite  as  complements  of  the 
Darwinian  theory.  I  will  now  conclude  by  briefly 
summarizing  these  matters  of  general  principle  in  the 
form  of  twelve  sequent  propositions.  And,  in  doing 
so,  I  may  ask  it  to  be  noticed  that  the  system  which 
these  propositions  serve  to  express  may  now  claim, 
at  the  least,  to  be  a  strictly  logical  system.  For  the 
fact  that,  not  merely  in  its  main  outlines,  but  likewise 
in  its  details,  it  has  been  independently  constructed 
by  Mr.  Gulick,  proves  at  any  rate  this  much ;  seeing 
that,  where  matters  of  such  intricacy  are  concerned, 
nothing  but  accurate  reasoning  from  a  common 
foundation  of  data  could  possibly  have  yielded  so 
exact  an  agreement.  The  only  difference  between  us 
is,  that  Mr.  Gulick  has  gone  into  much  further  detail 
than  I  have  ever  attempted  in  the  way  of  classifying 
the  many  and  varied  forms  of  isolation  ;  while  I  have 
laid  more  special  stress  upon  the  physiological  form, 
and  found  in  it  what  appears  to  me  a  satisfactory 
solution  of  "the  greatest  of  all  the  difficulties  in  the 
way  of  accepting  the  theory  of  natural  selection  as 
a  complete  explanation  of  the  origin  of  species  "- 
namely,  "the  remarkable  difference  between  varieties 
and  species  when  crossed." 


GENERAL  CONCLUSIONS. 

1.  NATURAL  SELECTION  is  PRIMARILY  A  THEORY 
OF  THE  CUMULATIVE  DEVELOPMENT  OF  ADAPTA- 
TIONS WHEREVER  THESE  OCCUR ;  AND  THEREFORE 
IS  ONLY  INCIDENTALLY,  OR  LIKEWISE,  A  THEORY 
OF  THE  ORIGIN  OF  SPECIES  IN  CASES  WHERE  ALLIED 
SPECIES  DIFFER  FROM  ONE  ANOTHER  IN  RESPECT  OF 
PECULIAR  CHARACTERS,  WHICH  ARE  ALSO  ADAPTIVE 
CHARACTERS. 

2.  HENCE,  IT   DOES   NOT  FOLLOW   FROM   THE 
THEORY    OF    NATURAL    SELECTION     THAT     ALL 
SPECIES— MUCH  LESS  ALL  SPECIFIC  CHARACTERS- 
MUST  NECESSARILY  HAVE  OWED  THEIR  ORIGIN  TO 
NATURAL  SELECTION  ;  SINCE  IT  CANNOT  BE  PROVED 
DEDUCTIVELY  FROM  THE  THEORY  THAT  NO  "  MEANS 
OF  MODIFICATION  "  OTHER  THAN  NATURAL  SELEC- 
TION   IS  COMPETENT  TO   PRODUCE  SUCH  SLIGHT 
DEGREES  OF   MODIFICATION   AS   GO   TO   CONSTI- 
TUTE DIAGNOSTIC  DISTINCTIONS  BETWEEN  CLOSELY 
ALLIED  SPECIES;   WHILE,  ON  THE  OTHER  HAND, 
THERE  IS  AN  OVERWHELMING  MASS  OF  EVIDENCE 
TO  PROVE  THE  ORIGIN  OF  "A  LARGE  PROPORTIONAL 
NUMBER  OF  SPECIFIC  CHARACTERS"  BY  CAUSES  OF 
MODIFICATION  OTHER  THAN  NATURAL  SELECTION. 


General  Conclusions.  145 

3.  THEREFORE,    AND    UPON    THE    WHOLE,    AS 
DARWIN    so    EMPHATICALLY    HELD,    "NATURAL 

SELECTION  HAS  BEEN  THE  MAIN,  BUT  NOT  THE 
EXCLUSIVE  MEANS  OF  MODIFICATION." 

4.  EVEN    IF   IT   WERE   TRUE    THAT   ALL   SPECIES 
AND  ALL  SPECIFIC  CHARACTERS  MUST  NECESSARILY 
OWE    THEIR    ORIGIN    TO    NATURAL    SELECTION,    IT 
WOULD    STILL  REMAIN  ILLOGICAL  TO  DEFINE   THE 
THEORY  OF  NATURAL  SELECTION  AS  INDIFFERENTLY 
A    THEORY  OF  SPECIES  OR  A   THEORY  OF  ADAPTA 
TIONS;    FOR,  EVEN   UPON   THIS  ERRONEOUS  SUPPO- 
SITION,    SPECIFIC      CHARACTERS     AND     ADAPTIVE 
CHARACTERS    WOULD    REMAIN    VERY    FAR    INDEED 
FROM   BEING  CONTERMINOUS — MOST  OF  THE   MORE 
IMPORTANT       ADAPTATIONS       WHICH       OCCUR      IN 
ORGANIC   NATURE   BEING  THE   COMMON    PROPERTY 
OF  MANY  SPECIES. 

5.  IN    NO   CASE   CAN   NATURAL  SELECTION  HAVE 
BEEN      THE      CAUSE      OF       MUTUAL      INFERTILITY 
BETWEEN  ALLIED,  OR  ANY  OTHER,  SPECIES— «.  €.  OF 
THE     MOST     GENERAL     OF     ALL    "SPECIFIC     CHAR- 
ACTERS." 

6.  WITHOUT  ISOLATION,  OR  THE  PREVENTION  OF 
FREE  INTERCROSSING,  ORGANIC  EVOLUTION  IS  IN 
NO  CASE  POSSIBLE.    THEREFORE,  IT  IS  ISOLATION 

THAT     has     BEEN      "THE     EXCLUSIVE     MEANS     OF 

MODIFICATION,"  OR,  MORE  CORRECTLY,  THE  UNI- 
VERSAL CONDITION  TO  IT.  THEREFORE,  ALSO, 
HEREDITY  AND  VARIABILITY  BEING  GIVEN,  THE 
WHOLE  THEORY  OF  ORGANIC  EVOLUTION  BECOMES 
A  THEORY  OF  THE  CAUSES  AND  CONDITIONS  WHICH 
LEAD  TO  ISOLATION. 

III.  L 


146         Darwin,  and  after  Darwin. 

7.  ISOLATION    MAY    BE    EITHER    DISCRIMINATE 
OR    INDISCRIMINATE.      WHEN    DISCRIMINATE,    IT 

HAS  REFERENCE  TO  RESEMBLANCES  BETWEEN  IN- 
DIVIDUALS CONSTITUTING  THE  ISOLATED  COLONY 
OR  GROUP;  WHEN  INDISCRIMINATE,  IT  HAS  NO 
SUCH  REFERENCE.  IN  THE  FORMER  CASE  THERE 
ARISES  HOMOGAMY,  AND  IN  THE  LATTER  CASE 

THERE  ARISES  APOGAMY. 

8.  EXCEPT  WHERE   VERY   LARGE   POPULATIONS 

ARE       CONCERNED,       INDISCRIMINATE        ISOLATION 

ALWAYS  TENDS  TO  BECOME  INCREASINGLY  DISCRIM- 
INATE ;  AND,  IN  THE  MEASURE  THAT  IT  DOES  SO, 
APOGAMY  PASSES  INTO  HOMOGAMY,  BY  VIRTUE  OF 
INDEPENDENT  VARIABILITY. 

9.  NATURAL  SELECTION  is  ONE  AMONG  MANY 

OTHER  FORMS  OF  DISCRIMINATE  ISOLATION,  AND 
PRESENTS  IN  THIS  RELATION  THE  FOLLOWING 
PECULIARITIES: — (a)  THE  ISOLATION  IS  WITH 
REFERENCE  TO  SUPERIORITY  OF  FITNESS;  (b)  IS 
EFFECTED  BY  DEATH  OF  THE  EXCLUDED  INDI- 
VIDUALS ;  AND  (c)  UNLESS  ASSISTED  BY  SOME 
OTHER  FORM  OF  ISOLATION,  CAN  ONLY  EFFECT 
MONOTYPIC  AS  DISTINGUISHED  FROM  POLYTYPIC 
EVOLUTION. 

10.  IT  IS  A  GENERAL  LAW  OF  ORGANIC  EVOLUTION 
THAT    THE    NUMBER    OF    POSSIBLE    DIRECTIONS    IN 
WHICH    DIVERGENCE    MAY    OCCUR     CAN    NEVER    BE 
MORE   THAN  EQUAL  TO  THE  NUMBER  OF   CASES  OF 
EFFICIENT  ISOLATION  ;     BUT,  EXCEPTING  NATURAL 
SELECTION,    ANY    ONE    FORM   OF    ISOLATION     NEED 
NOT     NKCESSARILY     REQUIRE     THE     CO-OPERATION 


General  Conclusions.  147 

OF  ANOTHER  FORM  IN  ORDER  TO  CREATE  AN 
ADDITIONAL  CASE  OF  ISOLATION,  OR  TO  CAUSE 
POLYTYPIC  AS  DISTINGUISHED  FROM  MONOTYPIC 
EVOLUTION. 

11.  WHERE  COMMON  AREAS  AND  POLYTYPIC  EVO- 
LUTION ARE  CONCERNED,  THE  MOST  GENERAL  AND 
MOST    EFFICIENT    FORM    OF    ISOLATION    HAS    BEEN 
THE     PHYSIOLOGICAL,    AND     THIS     WHETHER     THE 
MUTUAL  INFERTILITY  HAS  BEEN  THE  ANTECEDENT 

OR  THE  CONSEQUENT  OF  MORPHOLOGICAL  CHANGES 
ON  THE  PART  OF  THE  ORGANISMS  CONCERNED,  AND 
WHETHER  OR  NOT  THESE  CHANGES  ARE  OF  AN 
ADAPTIVE  CHARACTER. 

12.  THIS    FORM     OF     ISOLATION— WHICH,     IN 
REGARD  TO  INCIPIENT  SPECIES,  I   HAVE  CALLED 
PHYSIOLOGICAL    SELECTION — MAY    ACT    EITHER 

ALONE  OR  IN  CONJUNCTION  WITH  OTHER  FORMS  OF 
ISOLATION  ON  COMMON  AREAS:  IN  THE  FORMER 
CASE  ITS  AGENCY  IS  OF  MOST  IMPORTANCE  AMONG 
PLANTS  AND  THE  LOWER  CLASSES  OF  ANIMALS  ; 
IN  THE  LATTER  CASE  ITS  IMPORTANCE  CONSISTS 
IN  ITS  GREATLY  INTENSIFYING  THE  SEGREGATIVE 
POWER  OF  WHATEVER  OTHER  FORM  OF  ISOLATION 
IT  MAY  BE  WITH  WHICH  IT  IS  ASbUCIAiED. 


APPENDICES 


APPENDIX  A. 

MR.  GULICK'S    CRITICISM    OF    MR.   WALLACE'S    VIEWS   ON 
PHYSIOLOGICAL  SELECTION. 

I  HAVE  received  from  Mr.  Gulick  the  results  of  his 
consideration  of  Mr.  Wallace's  criticism.  As  these  results 
closely  resemble  those  which  I  have  myself  reached,  and 
as  they  were  independently  worked  out  on  the  other  side  of 
the  globe,  I  deem  it  desirable  to  publish  them  here  for  the 
sake  of  comparison. 

In  his  covering  letter  Mr.  Gulick  writes : — 

Mr.  Wallace  has  most  certainly  adopted  the  fundamental  prin- 
ciples of  our  theory,  and  in  an  arbitrary  way  attempted  to  claim 
the  results  produced  by  these  principles  as  the  effects  of  natural 
selection.  He  takes  our  principles,  which  in  the  previous 
chapter  he  has  combated  ;  but  he  makes  such  disjointed  use  of 
them  that  I  am  not  willing  to  recognize  his  statement  as  an 
intelligible  exposition  of  our  theory.  ...  I  have  endeavoured  to 
indicate  at  what  points  Mr.  Wallace  has  deserted  his  own  prin- 
ciples, and  at  what  points  he  has  failed  to  make  the  best  use  of 
ours.  To  bring  out  these  points  distinctly  has  been  no  easy 
task;  but  if  you  regard  this  paper  on  The  Preservation  and  Ac- 
cumulation of  Cross-infertility  as  giving  any  help  in  elucidating 
the  true  principles,  and  in  showing  Mr.  Wallace's  position  in 
regard  to  them,  I  shall  be  satisfied.  Please  make  any  use  of  it 
that  may  seem  desirable,  and  then  forward  it  to  Prolessor 
Dana. 


152        Darwin,  and  after  Darwin. 

The  following  is  a  general  summary  of  Mr.  Gulick's 

results: — 

Mr.  Wallace's  criticism  of  the  theory  of  Physiological  Selec- 
tion is  unsatisfactory ;  (i)  because  he  has  accepted  the  funda- 
mental principle  of  that  theory  on  pages  173-9,  in  that  he 
maintains  that  without  the  cross-infertility  the  incipient  species 
there  considered  would  be  swamped ;  (2)  because  he  assumes 
that  physiological  selection  pertains  simply  to  the  infertility 
of  first  crosses,  and  has  nothing  to  do  with  the  infertility  of 
mongrels  and  hybrids;  (3)  because  he  assumes  that  infertility 
between  first  crosses  is  of  rare  occurrence  between  species  of 
the  same  genus,  ignor  ng  the  fact  that  in  many  species  of  plants 
the  pollen  of  the  species  is  pre-potent  on  the  stigma  of  the  same 
species  when  it  has  to  compete  with  the  pollen  of  other  species 
of  the  same  genus ;  (4)  because  he  not  only  ignores  Mr.  Romanes' 
statement  that  cross-infertility  often  affects  "a  whole  race  or 
strain,"  but  he  gratuitously  assumes  that  the  theory  of  Physio- 
logical Selection  excludes  this  "racial  incompatibility"  (which 
Mr.  Romanes  maintains  is  the  more  probable  form),  and  bases  his 
computation  on  the  assumption  that  the  cross-infertility  is  not 
associated  with  any  other  form  of  segregation ;  (5)  because  he 
claims  to  show  that  "all  infertility  not  correlated  with  some 
useful  variation  has  a  constant  tendency  to  effect  its  own 
elimination,"  while  his  computation  only  shows  that,  if  the  cross- 
infertility  is  not  associated  with  some  form  of  positive  segre- 
gation, it  will  disappear1;  and  (6)  because  he  does  not  observe 
that  the  positive  segregation  may  be  secured  by  the  very  form  of 
the  physiological  incompatibility.  .  .  .  Without  here  entering 
into  any  computation,  it  is  evident  that,  e.  g.  the  prepotency  of 
pollen  of  each  kind  with  its  own  kind,  if  only  very  slight,  will 
prevent  cross-fertilization  as  effectually  as  a  moderate  degree  of 
instinctive  preference  in  the  case  of  an  animal. 

1  "  Positive  segregation "  is  Mr.  Gulick's  term  for  forms  of  homo- 
gamy  other  than  that  which  is  due  to  selective  fertility.  Of  these  other, 
or  "  positive  "  forms,  natural  selection  is  one ;  but  as  it  is  far  from 
being  the  only  one,  the  criticism  points  out  that  utility  is  not  the 
only  conserving  principle  with  which  selective  fertility  may  be  asso- 
ciated. 


Appendix  A.  153 

The  paper  likewise  indicates  a  point  which,  in  studying 
Mr.  Wallace's  theory,  I  have  missed.  It  will  be  remem- 
bered that  the  only  apparent  difference  between  his  theory 
and  mine  has  been  shown  to  consist  in  this — that  while 
I  was  satisfied  to  state,  in  a  general  way,  that  natural  selec- 
tion is  probably  able  to  increase  a  selective  fertility  which 
has  already  been  begun  by  other  causes,  Mr.  Wallace 
has  sought  to  exhibit  more  in  detail  the  precise  conditions 
under  which  it  can  do  so.  Now,  Mr.  Gulick  shows  that 
the  particular  conditions  which  Mr.  Wallace  describes,  even 
if  they  do  serve  to  promote  an  increase  of  cross-infertility, 
are  conditions  which  preclude  the  possibility  of  natural  selec- 
tion coming  into  play  at  all.  So  that  if,  under  these  parti- 
cular conditions,  a  further  increase  of  cross-infertility  does 
take  place,  it  does  not  take  place  in  virtue  of  natural  selection. 
To  me  it  appears  that  this  criticism  is  sound;  and,  if  so, 
it  disposes  of  even  the  one  very  subordinate  addition  to 
our  theory  which  Mr.  Wallace  "  claims "  as  the  most 
"  distinctive  "  part  of  his. 

The  following  is  the  criticism  in  question : — 

On  pages  173-186  Mr.  Wallace  maintains  that  "Natural 
selection  is,  in  some  probable  cases  at  all  events,  able  to 
accumulate  variations  in  infertility  between  incipient  species" 
(p.  174) ;  but  his  reasoning  does  not  seem  to  me  conclusive. 
Even  if  we  grant  that  the  increase  of  this  character  [cross- 
infertility]  occurs  by  the  steps  which  he  describes,  */  is  not 
a  process  of  accumulation  by  natural  selection.  In  order  to  be  a 
means  of  cumulative  modification  of  varieties,  races,  or  species, 
selection,  whether  artificial  or  adaptational  [i.e.  natural],  must 
preserve  certain  forms  of  an  intergenerating  stock,  to  the 
exclusion  of  other  forms  of  the  same  stock.  Progressive 
change  in  the  size  of  the  occupants  of  a  poultry-yard  may  be 
secured  by  raising  only  bantams  the  first,  only  common  fowls 
the  second,  and  only  Shanghai  fowls  the  third  year ;  but  this 
is  not  the  form  of  selection  that  has  produced  the  different 
races  of  fowls.  So  in  nature,  rats  may  drive  out  and  supplant 


154        Darwin,  and  after  Darwin. 

mice ;  but  this  kind  of  selection  modifies  neither  rats  nor  mice. 
On  the  other  hand,  if  certain  variations  of  mice  prevail  over 
others,  through  their  superior  success  in  escaping  their  pur- 
suers, then  modification  begins.  Now,  turning  to  page  175,  we 
find  that,  in  the  illustrative  case  introduced  by  Mr.  Wallace, 
the  commencement  of  infertility  between  the  incipient  species 
is  in  the  relations  to  each  other  of  two  portions  of  a  species 
that  are  locally  segregated  from  the  rest  of  the  species,  and 
partially  segregated  from  each  other  by  different  modes  of 
life.  These  two  local  varieties,  being  by  the  terms  of  his 
supposition  better  adapted  to  the  environment  than  the  freely 
interbreeding  forms  in  other  parts  of  the  general  area,  increase 
till  they  supplant  these  original  forms.  Then,  in  some  limited 
portion  of  the  general  area,  there  arise  two  still  more  divergent 
forms,  with  greater  mutual  infertility,  and  with  increased  adap- 
tation to  the  environment,  enabling  them  to  prevail  throughout 
the  whole  area.  The  process  here  described,  if  it  takes  place, 
is  not  modification  by  natural  selection. 

On  the  other  hand,  it  is  modification  by  physiological 
selection.  For,  among  the  several  other  forms  of  isolation 
which  are  called  into  requisition,  the  physiological  (i.e. 
ever  accumulating  cross-infertility)  is  supposed  to  play  an 
important  part.  That  the  modification  is  not  modification 
by  natural  selection  may  perhaps  be  rendered  more 
apparent  by  observing,  that  in  as  far  as  any  other  mode 
of  isolation  is  involved  or  supposed,  so  far  is  the  possible 
agency  of  natural  selection  eliminated  as  between  the  two 
or  more  otherwise  isolated  sections  of  a  species ;  and  yet  it  is 
modes  of  isolation  other  than  that  furnished  by  natural  selec- 
tion (i.e.  perishing  of  the  less  fit),  that  Mr.  Wallace  here 
supposes  to  have  been  concerned — including,  as  I  have 
before  shown,  the  physiological  form,  to  which,  indeed,  he 
really  assigns  most  importance  of  all.  Or,  as  Mr.  Gulick 
states  the  matter  in  his  independent  criticism: — 

In  the  supposed  case  pictured  by  Mr.  Wallace,  the  principle 
by  which  the  two  segregating  forms  are  kept  from  crossing, 


Appendix  A.  155 

and  so  are  eventually  preserved  as  permanently  distinct  forms, 
is  no  other  than  that  which  Mr.  Romanes  and  myself  have 
discussed  under  the  terms  Physiological  Selection  and  Segregate 
Fecundity.  Not  only  is  Mr.  Wallace's  exposition  of  the  diverg- 
ence and  the  continuance  of  the  same  in  accord  with  these 
principles  which  he  has  elsewhere  rejected,  but  his  whole 
exposition  is  at  variance  with  his  own  principle,  which,  in  the 
previous  chapter,  he  vigorously  maintains  in  opposition  to  my 
statement  that  many  varieties  and  species  of  Sandwich  Island 
land  molluscs  have  arisen,  while  exposed  to  the  same  environment, 
in  the  isolated  groves  of  the  successive  valleys  of  the  same 
mountain  range.  If  he  adhered  to  his  own  theory,  "  the  greater 
infertility  between  the  two  forms  in  one  portion  of  the  area" 
would  be  attributed  to  a  difference  between  the  environment  pre- 
sented in  that  portion  and  that  presented  in  the  other  portions ; 
and  the  difficulty  would  be  to  consistently  show  how  this 
greater  infertility  could  continue  unabated  when  the  varieties 
thus  characterized  spread  beyond  the  environment  on  which 
the  character  depends.  But,  without  power  to  continue,  the 
process  which  he  describes  would  not  take  place.  Therefore,  in 
order  to  solve  the  problem  of  the  origin  and  increase  of 
infertility  between  species,  he  tacitly  gives  up  his  own  theory, 
and  adopts  not  only  the  theory  of  Physiological  Selection  but 
that  of  Intensive  Segregation1  through  Isolation,  though  he 
still  insists  on  calling  the  process  natural  selection ;  for  on 
page  183  he  says,  "No  form  of  infertility  or  sterility  between 
the  individuals  of  a  species  can  be  increased  by  natural  selec- 
tion unless  correlated  with  some  useful  variation,  while  all 
infertility  not  so  correlated  has  a  constant  tendency  to  effect 
its  own  elimination. "  Even  this  claim  he  seems  to  unwittingly 
abandon  when  on  page  184  he  says:  "The  moment  it  [a 
species]  becomes  separated  either  by  geographical  or  selective 
isolation,  or  by  diversity  of  station  or  of  habits,  then,  while 
each  portion  must  be  kept  fertile  inter  se,  there  is  nothing 
to  prevent  infertility  arising  between  the  two  separated 
portions." 

1  By  Intensive  Segregation  Mr.  Gnlick  means  what  I  have  called  Inde- 
pendent Variability. 


156         Darwin,  and  after  Darwin. 

The  criticism  proceeds  to  show  yet  further  inconsistencies 
and  self-contradictions  in  Mr.  Wallace's  treatment  of  this 
subject;  but  it  now  seems  needless  to  continue.  Nor, 
indeed,  should  I  have  quoted  this  much  but  for  the  sake 
of  so  fully  justifying  my  own  criticism  by  showing  the 
endorsement  which  it  has  received  from  a  completely  in- 
dependent examination. 


APPENDIX    B. 

AH  EXAMINATION  BJ  MR.  FLETCHER  MOULTON  OF  MR. 
WALLACE'S  CALCULATION  TOUCHING  THE  POSSIBILITY  OF 
PHYSIOLOGICAL  SELECTION  EVER  ACTING  ALONE. 

WE  have  seen  that  the  only  important  point  of  difference 
between  Mr.  Wallace's  more  recent  views  and  my  own  on 
the  problem  of  inter-specific  sterility,  has  reference  to  the 
question  whether  variations  in  the  way  of  cross-infertility  can 
ever  arise  and  act  "  alone,  in  an  otherwise  undifferentiated 
species,"  or  whether  they  can  never  so  arise  and  act.  It 
is  Mr.  Wallace's  opinion  that,  even  if  they  ever  do  arise 
alone,  at  all  events  they  can  never  act  in  differentiating  a 
specific  type,  seeing  that  the  chances  against  their  suitable 
mating  must  be  so  great :  only  if  they  be  from  the  first 
associated  with  some  other  form  of  homogamy,  which  will 
have  the  effect  of  determining  their  suitable  mating,  does 
he  think  that  they  can  act  in  the  way  supposed  by  our 
theory  of  "  selective  fertility " '.  On  the  other  hand,  as 

1  His  sentence,  "  all  fertility  not  correlated  with  some  use/it/  variation 
has  a  constant  tendency  to  effect  its  own  elimination,"  still  further 
restricts  the  possible  action  of  physiological  selection  to  cases  where  at 
least  one  of  the  other  forms  of  homogamy  with  which  it  is  associated  is 
natural  selection.  Or,  in  other  words,  it  is  represented  that  physiological 
selection  must  always  be  associated  with  natural  selection,  even  if  it  be 
likewise  associated  with  any  other  form  of  exclusive  breeding.  But  as 
this  further  limitation  appears  to  me  self-evidently  unjustifiable  (seeing 


158        Darwin,  'and  after  Darwin. 

previously  and  frequently  stated,  I  have  so  strong  a  belief 
in  the  segregating  power  of  physiological  selection,  or 
selective  fertility,  that  I  do  not  think  it  is  necessary  for 
this  principle  to  be  always  associated  with  some  other  form 
of  homogamy.  From  the  first,  indeed,  I  have  laid  great 
stress  (as,  also,  has  Mr.  Gulick)  on  the  re-enforcing  influence 
which  association  with  any  other  form  of  homogamy  must 
exercise  upon  the  physiological  form,  and  vice  versa ;  but 
I  have  also  said  that,  in  my  opinion,  the  physiological  form 
may  in  many  cases  be  able  to  act  entirely  alone,  or  without 
assistance  derived  from  any  other  source.  The  question 
here  is,  as  we  have  already  so  fully  seen,  a  question  of  but 
secondary  importance  ;  since,  whether  or  not  the  physio- 
logical form  of  homogamy  ever  acts  alone,  even  Mr.  Wallace 
now  allows,  or  rather  argues,  that  it  acts  in  combination  — 
and  this  so  habitually,  as  well  as  with  so  much  effect,  that  it 
constitutes  a  usual  condition  to  the  origination  of  species. 
Nevertheless,  although  the  only  relevancy  of  his  numerical 
computation  of  chances — whereby  he  thinks  that  he  over- 
turns my  theory  in  toto — is  such  relevancy  as  it  bears  to  this 
question  of  secondary  importance,  I  have  thought  it  desirable 
to  refer  the  question,  together  with  Mr.  Wallace's  views  upon 
it,  to  the  consideration  of  a  trained  mathematician. 

As  this  "subordinate  question"  depends  entirely  on 
numerical  computations  involving  the  doctrine  of  chances, 
I  should  first  of  all  like  to  remark,  that  in  reference  to 
biological  problems  of  the  kind  now  before  us,  I  do  not 
myself  attach  much  importance  to  a  merely  mathematical 
analysis.  The  conditions  which  such  problems  involve  are 
so  varied  and  complex,  that  it  is  impossible  to  be  sure  about 
the  validity  of  the  data  upon  which  a  mathematical  analysis  is 

that  utility  is  not  the  only  possible  means  of  securing  effective  isolation) 
I  here  neglect  It,  and  take  the  wider  ground  marked  ont  above.  It  is 
needless  to  say  that  this  is  giving  Mr.  Wallace  every  possible  advantage, 
by  not  holding  him  to  his  still  narrower  ground. 


Appendix,  B.  159 

founded.  Nevertheless,  for  the  sake  of  meeting  these 
criticisms  upon  their  own  ground,  I  will  endeavour  to  show 
that,  even  as  mathematical  calculations,  they  are  quite  un- 
trustworthy. And,  in  order  to  do  this  effectually,  I  will  quote 
the  results  of  a  much  more  competent,  as  well  as  a  much  more 
thorough,  inquiry.  I  applied  to  Mr.  Moulton  for  this 
purpose,  not  only  because  he  is  one  of  the  ablest  mathe- 
maticians of  my  acquaintance  ;  but  also  because  his  interest 
in  biology,  and  his  knowledge  of  Darwinian  literature, 
render  him  well  fitted  to  appreciate  exactly,  and  in  all  their 
bearings,  the  questions  which  were  submitted  to  his  con- 
sideration. I  need  only  add  that  his  examination  was 
completely  independent,  and  in  no  way  influenced  by  me. 
Having  previously  read  my  paper  on  Physiological  Selection, 
Mr.  Gulick's  paper  on  Divergent  Evolution,  and  Mr.  Wallace's 
book  on  Darwinism,  he  was  in  possession  of  all  the  materials ; 
and  I  merely  requested  the  favour  of  his  opinion  upon  the 
whole  case  from  a  mathematical  point  of  view.  The 
following  is  his  reply ;  and  I  give  it  in  txtenso,  because  it 
serves  to  place  in  another  light  some  of  the  general  considera- 
tions which  it  has  already  been  my  endeavour  to  present1. 

After  some  introductory  remarks  on  Mr.  Wallace's 
"adoption  of  the  theory  of  physiological  selection  pure 
and  simple,"  and  "  the  pure  caricature  of  it  which  he 
puts  forward  as"  mine,  the  letter  proceeds  thus: — 

The  reason  why  it  is  so  easy  to  attack  your  theory  is  that 
it  is  so  easy  to  confuse  the  survival  of  an  individual  with  the 

1  In  our  Nature  correspondence  of  1890-1891,  Mr.  Wallace  remarked  : 
"  If  Dr.  Romanes  will  carefully  work  out  numerically  (as  I  have 
attempted  to  do)  a  few  cases  showing  the  preservative  and  accumulative 
agency  of  pure  physiological  selection  within  an  otherwise  nndifferentiated 
species,  he  will  do  more  for  his  theory  than  volumes  of  general  disquisi- 
tion or  any  number  of  assertions  that  it  does  possess  this  power." 
Several  months  before  this  was  written  I  had  already  in  my  hands 
Mr.  Moulton's  letter,  with  its  accompanying  calculations. 


160        Darwin,  and  after  Darwin. 

survival  of  a  peculiarity  of  type.  No  one  has  ever  said  that 
an  individual  is  assisted  by  the  possession  of  selective  fer- 
tility :  that  is  a  matter  which  cannot  affect  his  chance  of  life. 
Nor  has  any  one  said  that  the  possession  of  selective  fertility 
in  an  individual  will  of  itself  increase  the  chance  of  his  having 
Progeny  that  will  survive,  and  in  turn  become  the  progenitors 
of  others  that  will  survive.  Taken  by  itself,  the  fact  that  an 
individual  is  capable  of  fertility  with  some  only  of  the  oppo- 
site sex  lessens  the  chance  of  his  having  progeny.  Whether 
or  not  he  is  more  or  less  favourably  situated  than  his  con- 
freres for  the  battle  of  life  must  be  decided  by  the  total  sum 
of  his  peculiarities ;  and  the  question  whether  or  not  this 
selective  fertility  will  be  a  hindrance  must  be  decided  by 
considerations  depending  on  the  other  peculiarities  associated 
with  it. 

But  when  we  come  to  consider  the  survival  or  permanence 
of  a  type  or  peculiarity,  the  case  is  quite  different.  It  then 
becomes  not  only  a  favourable  circumstance,  but,  in  my  opinion, 
almost  a  necessary  condition,  that  the  peculiarity  should  be 
associated  with  selective  fertility1. 

Take  the  case  of  the  Jews.  I  don't  think  that  intermarriage 
with  other  nations  would  lessen  their  fertility,  or  diminish  the 
number  of  their  progeny  ;  nor  is  there  any  reason  to  think  that 
this  progeny  would  be  unequal  to  the  struggle  for  existence. 
But  no  one  doubts  that  the  abandonment  of  their  voluntary 
isolation  (which  operates  so  far  as  this  is  concerned  as  a  selec- 
tive fertility),  would  lead  to  the  disappearance  of  the  familiar 
Jewish  type.  All  the  world  would  get  some  of  it ;  but  as  a  whole 
it  would  be  "swamped." 

Now  although  no  doubt  Wallace  would  admit  all  this,  he 
fails  to  give  it  the  weight  it  ought  to  have.  In  discussing  the 
question  of  its  operation  he  considers  too  exclusively  the  case 
of  the  individual. 

Of  course,  a  type  can  only  be  perpetuated  through  the  medium 
of  individuals,  and  all  that  his  argument  amounts  to  is,  that 

1  As,  for  example,  in  the  case  of  sexuality  in  general.  It  is  not  to 
the  advantage  of  such  individual  male  Arthropoda  as  perish  after  the 
performance  of  the  sexual  act  that  they  should  perform  it;  but  its  per- 
formance is  necessary  for  the  perpetuation  of  their  species.— G.  J.  R. 


Appendix  B.  161 

selective  fertility  would  be  so  fatal  to  individuals  that  no  type 
which  presents  it  could  be  formed  or  perpetuated— a  conclu- 
sion which  is  not  only  absurd  in  itself,  but  contradicted  by 
his  own  subsequent  adoption  of  your  theory.  Besides,  apart 
from  calculations  (with  which  I  will  deal  when  I  write  next), 
such  reasoning  brings  its  own  refutation.  Selective  fertility  is 
not  in  the  same  category  as  some  of  the  other  influences  to 
which  an  important  share  has  been  ascribed  in  the  formation 
of  the  existing  types.  //  exists  as  a  recognized  phenomenon. 
Hence  all  these  numerical  proofs  that  it  would  lead  to  extinc- 
tion, because  it  is  so  disadvantageous  to  the  possessor,  prove 
too  much.  They  would  show  that  the  degree  of  selective 
fertility  which  so  frequently  characterizes  species  is  a  most 
onerous  gift  ;  and  that,  were  it  not  present,  there  would  be 
a  vastly  increased  chance  of  fertility,  which  would  render  the 
races  fitter  and  lead  to  their  increased  survival.  Why  then 
has  it  not  been  got  rid  of? 

The  two  answers  which  no  doubt  would  be  given  seem  to 
me  to  support  rather  than  to  make  against  your  theory.  In 
the  first  place,  Wallace  might  say  that  this  infertility  is  an 
advantage  because  it  keeps  pure  a  type  which  is  specially 
fitted  to  its  surroundings,  as  shown  by  its  continued  existence. 
But  if  this  be  so,  and  it  is  necessary  to  protect  the  developed 
type,  how  much  more  necessary  to  protect  the  incipient  type ! 
In  the  second  place,  he  might  say  that  this  selective  fertility 
is  not  so  disadvantageous  when  the  species  has  been  formed, 
because  the  individual  can  choose  his  mate  from  his  like ; 
whereas,  when  it  is  beginning  to  be  formed,  he  must  mate 
blindly,  or  without  what  you  call  "  psychological  selection.1' 
But  this  seems  to  me  to  be  wholly  inapplicable  to  at  least  half 
the  animal,  and  to  all  the  vegetable  kingdom.  Moreover,  with  re- 
gard to  the  other  half  of  the  animal  kingdom,  it  merely  raises  the 
question,— How  soon  will  such  an  incipient  type  recognize  itself? 
Seeing  it  is  probable  that  many  families  [broods]  will  belong  to 
the  same  [incipient]  type,  I  should  not  be  surprised  if  it  were 
found  that  this  sexual  recognition  and  preference  sets  in  very 
early. 

But  this  leads  me  to  the  question  of  your  letter.  I  under- 
stand you  to  want  me  to  examine  and  criticize  the  attempted 

III.  M 


162        Darwin,  and  after  Darwin. 

numerical  arguments  against  or  for  your  theory.  Now  it  seems 
to  me  that  it  will  be  best  to  take,  in  the  first  instance,  the 
vegetable  kingdom,  and  with  regard  to  it  I  cannot  see  how 
there  can  be  any  numerical  argument  against  the  theory.  For 
we  often  have  species  side  by  side  with  others  nearly  allied, 
but  much  more  numerous.  The  condition  of  these  is  precisely- 
analogous  to  that  of  your  incipient  species.  They  are  exposed 
to  fertilization  from,  say,  ten  times  as  numerous  individuals  of 
the  allied  species.  They  reject  this  in  favour  of  that  from  the 
relatively  few  individuals  of  their  own.  Yet  the  two  species 
are  in  competition.  I  could  go  through  the  numerical  argu- 
ments of  your  assailant  word  for  word,  applying  them  to 
such  a  case  as  this,  and  they  would  triumphantly  show  that 
the  specific  fertility  of  the  rarer  kind  would  lead  to  its  certain 
extinction.  Yet  we  know  that  this  is  not  so. 

Indeed,  the  too  triumphant  character  of  the  logic  used  against 
you  seems  to  me  to  be  capable  of  being  turned  to  your  use. 
If  cross-infertility  is  so  intensely  disadvantageous  to  the  indi- 
viduals presenting  it,  it  cannot  have  been  that  which  made 
these  individuals  and  their  progeny  survive.  It  is  therefore 
a  burden  which  they  have  carried.  But  we  find  that  it  is 
more  or  less  present  in  all  the  closely  allied  types  that  occur 
on  common  areas :  therefore  it  must  be  a  necessary  feature 
in  the  formation  of  such  types ;  for  it  cannot  be  an  accident 
that  it  is  present  in  so  many.  In  other  words,  it  must  be 
the  price  which  the  individual  and  his  progeny  pay  for  their 
formation  into  a  type.  And  this  is  your  theory  pure  and 
simple. 

The  more  I  consider  the  matter,  the  more  I  feel  that  it  is 
impossible  to  decide  as  to  the  sufficiency  of  selective  fertility 
to  explain  the  formation  of  species,  if  we  consider  merely  the 
effect  it  would  have  on  the  number  of  individuals,  as  con- 
trasted with  what  it  would  be  if  no  such  peculiarity  bad  de- 
veloped itself.  Indeed,  I  may  say  that  on  pondering  over 
the  matter  I  have  come  to  the  conclusion,  that  mere  fertility 
is  probably  a  comparatively  unimportant  factor  in  the  preser- 
vation of  the  species,  after  a  certain  sufficient  degree  of  fertility 
is  attained.  I  do  not  wish  to  be  misunderstood.  To  a  certain 
point  fertility  is  not  only  advantageous  but  necessary,  in 


Appendix  B.  163 

order  to  secure  survival  of  the  type;  but  I  feel  that  little 
reliance  can  be  placed  on  calculations  based  on  the  numerical 
co-efficient  of  fertility  (i.  e.  the  ratio  of  the  number  of  offspring 
to  the  number  of  parents)  in  determining  the  relative  chance  of 
type-survival. 

Take,  for  instance,  the  oak  tree.  It  produces  thousands  of 
acorns,  almost  the  whole  of  which  die  without  producing  any 
progeny.  Have  we  any  reason  to  believe  that  if  the  number 
of  acorns  borne  by  oak  trees  were  diminished,  even  so  much 
as  to  one-tenth,  the  race  of  oaks  would  perish  ?  It  may 
of  course  be  said  that,  if  all  other  things  are  equal,  the  pro- 
babilities of  survival  must  be  increased  by  increased  fertility 
of  this  kind ;  but  I  feel  convinced  that  when  numerical  fertility 
has  attained  to  a  high  point  in  circumstances  in  which 
actual  increase  of  the  race  cannot  take  place  to  any  substan- 
tial extent,  the  numerical  value  of  this  fertility  sinks  down 
into  a  factor  of  the  second  or  third  order  of  importance— that 
is  to  say,  into  the  position  of  a  factor  whose  effects  are  only  to 
be  considered  when  we  have  duly  allowed  for  the  full  effects 
of  all  the  main  factors.  Until  we  have  done  that,  we  gain 
little  or  nothing  in  the  way  of  accuracy  of  conclusion  by  taking 
into  consideration  the  minor  factors.  It  may  be  very  well  to 
neglect  the  effect  of  the  attraction  of  Jupiter  in  our  early  re- 
searches on  the  motion  01  the  Moon ;  and  our  doing  so  will 
not  prevent  the  results  being  approximate  and  having  consider- 
able value,  because  we  are  retaining  the  two  main  factors  that 
establish  the  motion,  viz.  the  effects  of  the  Earth  and  the  Sun. 
But  if  we  exclude  the  effect  of  one  of  these  main  factors,  our 
results  would  be  worthless ;  and  it  would  not  be  rendered  sub- 
stantially less  so  by  the  fact  that  we  had  taken  Jupiter  into 
account  in  arriving  at  them. 

You  must  not  imagine,  however,  that  I  think  it  wholly  profit- 
less to  see  whether  there  would  be  any  substantial  effect  on 
numerical  fertility  were  selective  fertility  to  manifest  itself.  But 
if  we  want  to  derive  any  assistance  from  calculation,  it  must 
be  by  applying  it  with  a  good  deal  more  precision  and  definite- 
ness  than  anything  that  Wallace  shows.  And,  in  the  first 
place,  it  is  useless  to  confuse  the  vegetable  and  animal  kingdoms. 
In  the  former  you  have  union  unaffected  by  choice ;  in  the  latter, 
M  2 


164         Darwin,  and  after  Darwin. 

so  far  at  all  events  as  the  higher  animals  are  concerned,  you 
have  "  psychological  selection."  In  order  to  give  you  a  speci- 
men of  what  can  safely  be  done  by  calculation  if  you  take 
a  problem  of  sufficient  definiteness,  I  have  chosen  the  case  of 
a  flowering  plant  in  which  a  certain  proportion  of  the  race 
have  developed  the  peculiarity  of  being  sterile  with  the  re- 
mainder, while  retaining  the  normal  fertility  of  the  race  in 
unions  among  themselves.  In  order  to  give  the  greatest  ad 
vantage  to  your  critics,  I  have  assumed  that  such  flowers  as 
possess  the  peculiarity  are  not  self-fertilizable ;  for  it  is  clear  that 
if  we  suppose  that  they  are  self-fertilizable,  the  fertility  need 
be  very  slightly  affected. 

As  I  have  excluded  self-fertilization,  it  is  necessary,  if  we  are 
to  get  any  trustworthy  results,  that  one  should  consider  the 
mode  in  which  fertilization  will  be  produced.  I  have  taken 
the  case  of  fertilization  by  insects,  and  have  assumed  that  each 
flower  is  visited  a  certain  number  of  times  by  insects  during 
the  period  when  fertilization  is  possible ;  and,  further,  that  the 
insects  which  visit  it  have  on  the  average  visited  a  certain 
number  of  flowers  of  the  same  species  before  they  came  there. 
Of  course  nothing  but  observation  can  fix  these  latter  numbers ; 
but  I  should  not  be  surprised  at  finding  that  they  are  of 
considerable  magnitude 1.  In  order  to  make  the  results  a  little 

1  In  this  anticipation  Mr.  Moulton  is  right.  The  well-known  botanist, 
Mr.  Bennett,  read  a  most  interesting  paper  on  the  subject  before  the 
British  Association  in  1881.  His  results  have  since  been  corroborated 
by  other  observers.  In  particular,  Mr.  R.  M.  Christy  has  recorded  the 
movements  of  76  insects  while  visiting  at  least  2.400  flowers.  {Entomo- 
logist, July  1883,  and  Zool.  Journal  Lin.  Soc.,  August  1883.)  The 
following  is  an  analysis  of  his  results.  In  the  case  of  butterflies,  in 
twelve  observations  on  nearly  as  many  species,  there  are  recorded 
altogether  99  visits  to  fifteen  species  of  flowers ;  and  of  these  99  visits  94 
were  constant  to  the  same  species,  leaving  only  5  visits  to  any  other, 
or  second  species.  In  the  case  of  the  hive-bee,  there  were  8  individuals 
observed  :  these  visited  altogether  258  flowers,  and  all  the  visits  paid  by 
the  same  individual  were  paid  to  the  same  species  in  each  of  the  eight 
cases.  Lastly,  as  regards  humble-bees,  there  were  altogether  obseived 
55  individuals  belonging  to  four  species.  These  paid  altogether  1751 
visits  to  94  species  of  flowers.  Of  these  1751  visits,  1605  were  paid  to 
one  species,  131  to  two  species,  16  to  three,  6  to  four,  and  i  to  five. 


Appendix  B.  165 

more  intelligible,  I  have  grouped  them  under  the  numbers  which 
represent  the  average  number  of  flowers  that  an  insect  visits 
in  a  journey.  This  is  a  little  more  than  twice  as  great  as  the 
number  which  represents  the  number  of  flowers  he  has  on  the 
average  visited  before  coming  to  the  individual  whose  fertility 
we  are  considering. 

I  send  you  the  formula  and  the  calculation  on  which  it  is 
based  in  an  Appendix ;  but  as  I  know  you  have  a  holy  horror 
of  algebraical  formulae,  I  give  you  here  a  few  numerical 
results. 

The  cases  I  have  worked  out  are  those  in  which  the  number 
of  insects  visiting  each  flower  is  5,  or  10,  or  15  ;  and  I  have 
also  taken  5,  10,  and  15,  to  represent  the  number  of  flowers 
which  an  insect  visits  each  journey.  This  makes  nine  cases 
in  all ;  and  I  have  applied  these  to  two  instances — viz.  one 
in  which  one-fifth  of  the  whole  race  have  developed  cross- 
infertility,  and  the  other  in  which  one-tenth  only  have  done  so. 
Taking  first  the  instance  where  one-fifth  have  developed  the 
peculiarity,  I  find  that  if  on  the  average  five  insects  visit 
a  flower,  and  each  insect  on  the  average  visits  five  flowers  on 
a  journey,  the  fertility  is  diminished  by  about  one-tenth.  If, 
however,  the  average  number  of  flowers  the  insect  visits  is  ten, 
the  reduction  of  fertility  is  less  than  one  per  cent  And  it 
becomes  inappreciable  if  the  average  number  is  fifteen.  If  on 
the  average  ten  insects  visit  each  flower,  then,  if  each  insect 
visits  on  the  average  five  flowers  on  a  journey,  the  reduction 
of  fertility  is  a  little  over  one  per  cent. ;  but  if  it  visits  ten  or 
fifteen  the  reduction  is  inappreciable.  If  fifteen  insects  visit  the 
flower  on  an  average,  then,  if  these  insects  on  the  average  visit 

A.dding  all  these  results  together,  we  find  that  75  insects  (butterflies 
and  bees)  visited  117  species  of  flowers:  of  these  visits,  1957  were 
constant  to  one  species  of  flower;  136  were  paid  also  to  a  second 
species,  16  also  to  a  third,  6  also  to  a  fourth,  and  i  also  to  a  fifth.  Or, 
otherwise  stated,  while  1957  were  absolutely  constant,  from  such  absolute 
constancy  there  were  only  159  deviations.  Moreover,  if  we  eliminate 
three  individual  humble-bees,  which  paid  nearly  an  equal  number  of  visits 
to  two  species  (and,  therefore,  would  have  ministered  to  the  work  of 
physiological  selection  almost  as  well  as  the  others),  the  159  deviations 
become  reduced  to  73,  or  about  four  per  cent,  of  the  whole. — G.  J.  R. 


i66        Darwin,  and  after  Darwin. 

five  or  more  flowers  on  a  journey,  the  reduction  of  fertility 
is  inappreciable. 

By  the  term  inappreciable  I  mean  that  it  is  not  substan- 
tially greater  than  one-tenth  of  one  per  cent— i.e.  not  more 
than  one-thousandth. 

Of  course,  if  the  proportion  of  individuals  acquiring  the 
peculiarity  is  less,  the  effect  on  the  fertility  under  the  above 
hypothesis  will  be  greater ;  and  it  will  not  be  counteracted  so 
fully  unless  the  number  of  insect  visits  is  larger,  or  unless  the 
insects  visit  more  flowers  on  a  journey.  Thus  if  only  one-tenth 
of  the  race  have  developed  the  peculiarity,  then,  if  each  flower 
is  visited  on  the  average  by  five  insects  who  visit  five  flowers 
on  each  trip,  the  fertility  will  be  reduced  about  one-third. 
If,  however,  the  insects  visit  on  the  average  ten  flowers  per 
trip,  it  will  be  only  diminished  about  one-tenth  ;  and  if  they 
visit  fifteen  on  each  trip,  it  will  be  only  diminished  about 
one-fortieth.  If  in  the  same  case  we  suppose  that  each 
flower  receives  ten  insect  visits,  then,  if  the  insects  visit  on  an 
average  five  flowers  per  trip,  the  fertility  will  be  diminished 
about  one-eighth.  If  they  visit  ten  on  a  trip,  it  will  be  dimi- 
nished about  one-hundredth,  and  the  diminution  is  inappreciable 
if  they  visit  fifteen  on  a  trip.  Similarly,  if  a  flower  receives 
fifteen  insect  visits,  the  diminution  is  about  one-twenty-fifth, 
if  insects  visit  on  the  average  five  flowers  on  a  trip ;  and  is 
inappreciable  if  they  visit  ten  or  fifteen. 

These  figures  will  show  you  that  it  is  exceedingly  possible 
that  a  peculiarity  like  this,  the  effect  of  which  at  first  sight 
would  seem  to  be  so  prejudicial  to  fertility,  may  in  fact  have 
little  or  no  influence  upon  it ;  and  if  you  set  against  this  the 
overwhelming  importance  of  such  a  peculiarity  in  segregating 
the  type  so  as  to  give  it  a  chance  of  becoming  a  fixed  species, 
you  will,  I  think,  feel  that  your  hypothesis  has  nothing  to 
fear  from  a  numerical  examination. 

I  have  not  examined  the  case  of  fertilization  by  other  means ; 
nor  have  I  examined  the  case  of  fertilization  in  animals,  where 
psychological  selection  can  come  in.  To  obtain  any  useful 
results,  one  would  have  to  consider  very  carefully  the  circum- 
stances of  each  case ;  and  at  present,  at  all  events,  I  do  not 
think  it  would  be  useful  to  do  so.  Nor  have  I  attempted  to 


Appendix  B.  167 

show  the  converse  of  the  problem  -  viz.  the  effect  of  swamping 
where  cross-fertilization  is  possible.  I  shall  be  very  glad  to 
examine  any  one  of  these  cases  if  you  want  me  to  do  so ; 
but  I  should  prefer  to  leave  it  until  I  hear  from  you  again. 

If  you  contrast  the  results  that  I  have  given  above  with 
those  given  on  pages  181  to  183  of  Wallace's  book,  you  will 
see  the  enormous  difference.  His  calculations  can  only  apply 
to  the  animal  kingdom  in  those  cases  in  which  there  is  only 
a  union  between  one  individual  of  each  sex  ;  and  before  you 
can  deal  with  the  question  of  such  animals,  you  will  have  to 
take  into  consideration  many  elements  besides  that  of  mere 
fertility,  if  you  wish  to  get  any  tolerably  accurate  result1. 

The  above  analysis  leaves  nothing  to  be  added  by  me. 
But,  in  conclusion,  I  may  once  more  repeat  that  the  particular 
point  with  which  it  is  concerned  is  a  point  of  very  subor- 
dinate importance.  For  even  if  Mr.  Wallace's  computation 
of  chances  had  been  found  by  Mr.  Moulton  to  have  been  an 
adequate  computation — and,  therefore,  even  if  it  had  been 
thus  proved  that  physiological  homogamy  must  always  be 
associated  with  some  other  form  of  homogamy  in  order  to 
produce  specific  divergence — still  the  importance  of  selective 
fertility  as  a  factor  of  organic  evolution  would  not  have 
been  at  all  diminished.  For  such  a  result  would  merely 
have  shown  that,  not  only  "  in  many  cases  "  (as  I  originally 
said),  but  actually  in  all  cases,  the  selective  fertility  which 
I  hold  to  have  been  so  generally  concerned  in  the  differentia- 
tion of  species  has  required  for  this  purpose  the  co-operation 
of  some  among  the  numerous  other  forms  of  homogamy. 
But  inasmuch  as,  by  hypothesis,  no  one  of  these  other  or 
co-operating  factors  would  of  itself  have  been  capable  of 
effecting  specific  divergence  in  any  of  the  cases  where  its 
association  with  selective  fertility  is  concerned,  the  mathe- 

1  Here  follows  the  Appendix  presenting  the  calculations  on  which  the 
above  results  are  founded  ;  but  it  seems  unnecessary  to  reproduce  it 
on  the  present  occasion. — G.  J.  R. 


168         Damvin,  and  after  Darwin. 

matical  proof  that  such  an  association  is  always — and  not 
merely  often — necessary,  would  not  have  materially  affected 
the  theory  of  the  origin  of  species  by  means  of  physiological 
selection.  We  have  now  seen,  however,  that  a  competent 
mathematical  treatment  proves  the  exact  opposite ;  and,  there- 
fore, that  Mr.  Wallace's  criticism  fails  even  as  regards  the 
very  subordinate  point  in  question. 


APPENDIX  C 

SOUK    EXTRACTS   FROM    THE    AUTHORS    NOTE-BOOKS. 

Bearing  of  Weismannism  on  Physiological  Selection. — If 
in  view  of  other  considerations  I  could  fully  accept  Professor 
Weismann's  theory  of  heredity,  it  would  appear  to  me  in  no 
small  measure  to  strengthen  my  own  theory  of  physiological 
selection.  For  Weismann's  theory  supposes  that  all  changes 
of  specific  type  must  have  their  origin  in  variations  of  a 
continuous  germ-plasm.  But  the  more  the  origin  of  species  is 
referred  directly  to  variations  arising  in  the  sexual  elements, 
the  greater  is  the  play  given  to  the  principles  of  physiological 
selection  ;  while,  on  the  other  hand,  the  less  standing-ground 
is  furnished  to  the  theory  that  cross-infertility  between  allied 
species  is  due  to  "external  conditions  of  life,"  "prolonged 
exposure  to  uniform  change  of  conditions,"  "structural 
modifications  re-acting  on  the  sexual  functions " ;  or,  in 
short,  that  "  somatogenetic "  changes  of  any  kind  can  of 
themselves  induce  the  "  blastogenetic "  change  of  cross- 
infertility  between  progeny  of  the  same  parental  stock. 

Cross-infertility  and  Diversity  of  Life. — Observe  that  one 
great  consequence  of  duly  recognizing  the  importance  of  inter- 
crossing is  indefinitely  to  raise  our  estimate  of  the  part  played 
by  the  principle  of  cross-infertility  in  diversifying  organic 
nature.  For  whenever  in  any  line  of  descent  the  bar  of 

1  Doctrine  of  Descent  and  Darwinism,  Eng.  trans,  p.  139. 


170         Darwin,  and  after  Darwin. 

sterility  arises,  there  the  condition  is  given  for  a  new  crop  of 
departures  (species  of  a  genus) ;  and  when  genera  are  formed 
by  the  occurrence  of  this  bar,  there  natural  selection  and  all 
other  equilibrating  causes  are  supplied  with  new  material  for 
carrying  on  adaptational  changes  in  new  directions.  Thus, 
owing  to  cross-infertility,  all  these  causes  are  enabled  to 
work  out  numberless  adaptations  in  many  directions  (i.  e. 
lines  of  descent)  simultaneously. 

Cross-infertility  and  Stability. — The  importance  of  sterility 
as  a  diagnostic  feature  is  obvious  if  we  consider  that  more 
than  any  other  feature  it  serves  to  give  stability  to  the  type ; 
and  unless  a  type  is  stable  or  constant,  it  cannot  be  ranked 
as  a  species.  That  Darwin  himself  attributes  the  highest 
importance  to  this  feature  as  diagnostic,  see  Forms  of  Flowers, 
pp.  58,  64. 

Cross-infertility  and  Specific  Differentiation.  —  In  their 
elaborate  work  on  the  many  species  of  the  genus  Hieracium, 
Nageli  and  Peter  are  led  to  the  general  conclusion  that  the 
best  defined  species  are  always  those  which  display  absolute 
sterility  inter  se ;  while  the  species  which  present  most 
difficulty  to  the  systematist  are  always  those  which  most 
easily  hybridize.  Moreover,  they  find,  as  another  general 
rule  applicable  to  the  whole  genus,  that  there  is  a  constant 
correlation  between  inability  to  hybridize  and  absence  of 
intermediate  varieties,  and,  conversely,  between  ability  to 
hybridize  and  the  presence  of  such  varieties. 

Cross-in/erti lily  in  Domesticated  Cattle. — Mr.J.W.Crompton, 
who  has  had  a  large  experience  as  a  professional  cattle- 
breeder,  writes  to  me  (March  2,  1887) — 

"That  form  of  barrenness,  very  common  in  some  districts, 
which  makes  heifers  become  what  are  called  'bullers'— that 
is,  irregularly  in  'season,'  wild,  and  failing  to  conceive— is 
certainly  produced  by  excess  of  iron  in  their  drinking-water, 
and  I  suspect,  also  by  a  deficiency  of  potash  in  the  soil." 


Appendix  C.  171 

He  also  informs  me  that  pure  white  beasts  of  either  sex 
are  so  well  known  by  experienced  breeders  to  be  comparatively 
infertile  together,  that  they  are  never  used  for  breeding 
purposes,  so  that  "in  some  parts  of  the  country,  where  a 
tendency  to  sterility  had  become  so  confirmed  in  the  white 
race  that  they  utterly  died  out,"  only  the  coloured  breeds  are 
now  to  be  found.  He  goes  on  to  say  that  if  "  a  lot  of  white 
heifers  were  put  to  a  lot  of  white  bulls,  I  think  you  would 
probably  get  a  fertile  breed  of  pure  white  cattle.  ...  I  think, 
in  short,  that  domestication  has  produced  just  what  your 
theory  suggests,  a  new  variety  inclined  to  prove  sterile  with 
its  parent  stock." 

Commenting  on  the  origin  of  domesticated  cattle,  Professor 
Oscar  Schmidt  remarks  (Doctrine  of  Descent,  p.  139) — 

"  Riitimeyer's  minute  researches  on  domestic  cattle  have  shown 
that,  in  Europe  at  least,  three  well-defined  species  of  the  diluvial 
period  have  contributed  to  their  formation — Bos  primigenius, 
longifrons,  and  frontosus.  These  species  once  lived  geogra- 
phically separate,  but  contemporaneously ;  and  they  and  their 
specific  peculiarites  have  perished,  to  rise  again  in  our  domestic 
races.  These  races  breed  together  with  unqualified  fertility. 
In  the  form  of  skull  and  horns  they  recall  one  or  other  of  the 
extinct  species ;  but  collectively  they  constitute  a  newmain  species. 
That  from  their  various  breeds,  the  three  or  any  one  of  the 
aboriginal  species  would  ever  emerge  in  a  state  of  pristine 
purity,  would  be  an  utterly  ludicrous  assertion." 

Now,  seeing  that  these  "  aboriginal  species,"  although  living 
"contemporaneously,"  were  "geographically  separate,"  we 
can  well  understand  that  their  divergence  of  type  from  a 
common  ancestor  did  not  require,  as  a  condition  to  their 
divergence,  that  any  cross-sterility  should  have  arisen  between 
them.  The  geographical  isolation  was  enough  to  secure 
immunity  from  mulual  intercrossing,  and  therefore,  as  our 
present  theory  would  have  expected  as  probable,  morpho- 
logical divergence  occurred  without  any  corresponding  physio- 


172         Darwin,  and  after  Darwin. 

logical  divergence,  as  must  almost  certainly  have  been  the 
case  if  such  polytypic  evolution  had  occurred  on  a  common 
area.  Indeed,  one  of  the  two  lines  of  experimental  verifica- 
tion of  our  theory  consists  in  selecting  cases  where  nearly 
allied  species  are  separated  by  geographical  barriers,  and 
proving  that,  in  such  cases,  there  is  no  cross-sterility. 

Fertility  of  Domesticated  Varieties, — Some  writers  have 
sought  to  explain  the  contrast  between  domesticated  varieties 
and  natural  species  in  respect  of  fertility  when  crossed,  by 
the  consideration  that  it  is  only  those  natural  species  which 
have  proved  themselves  so  far  flexible  as  to  continue  fertile 
under  changed  conditions  of  life  that  can  have  ever  allowed 
themselves  to  become  domesticated.  But  although  this 
condition  may  well  serve  to  explain  the  unimpaired  fertility 
under  domestication  of  such  species  as  for  this  very  reason  have 
ever  become  domesticated,  I  fail  to  see  how  it  explains  the 
further  and  altogether  different  fact,  that  this  fertility  continues 
unimpaired  between  all  the  newly  differentiated  morphological 
types  which  have  been  derived  from  the  original  specific  type. 
It  is  one  thing  that  this  type  should  continue  fertile  after 
domestication :  it  is  quite  another  thing  that  fertility  should 
continue  as  between  all  its  modified  descendants,  even 
although  the  amount  of  modification  may  extend  much 
further  than  that  which  usually  obtains  between  different 
natural  species. 

Testing  for  Cross-infertility  among  varieties  growing  on 
the  same  area  is  a  much  more  crucial  line  of  verification  than 
testing  for  unimpaired  fertility  between  allied  species  which 
occupy  different  areas,  because  while  in  the  former  case  we 
are  dealing  with  "  incipient  species  "  with  a  view  to  ascertain- 
ing whether  the  divergence  which  they  have  already  undergone 
is  accompanied  by  physiological  isolation,  in  the  latter  case 
we  can  never  be  sure  that  two  allied  species,  which  are  now 
widely  disconnected  geographically,  have  always  been  so 


Appendix  C.  173 

disconnected.  They  may  both  have  originated  on  the  same 
area;  or  one  may  have  diverged  from  the  other  before  it 
migrated  from  that  area  ;  or  even  if,  when  it  migrated,  it  was 
unchanged,  and  if  in  its  new  home  it  afterwards  split  into  two 
species  by  physiological  selection,  the  newer  species  would 
probably  prove  infertile,  not  only  with  its  parent  type,  but 
also  with  its  grand-parent  in  any  other  part  of  the  world. 

Seebohm  on  Isolation. — Seebohm  is  so  strongly  influenced 
by  the  difficulty  from  "  the  swamping  effects  of  free  intercross- 
ing," that  he  is  driven  by  it  to  adopt  Asa  Gray's  hypothesis 
of  variations  as  Ideological.  Indeed,  he  goes  as  far  as 
Wagner,  for  he  maintains  that  in  no  case  can  there  be 
divergence  or  multiplication  of  species  without  isolation. 
He  makes  the  important  statement  that  "the  more  the 
geographical  distribution  of  birds  is  studied,  the  more  doubtful 
it  seems  to  be  that  any  species  of  bird  has  ever  been  differen- 
tiated without  the  aid  of  geographical  isolation"  (Charadriidae, 
p.  17).  If  this  is  true,  it  makes  in  favour  of  physiological 
selection  by  showing  the  paramount  importance  of  the 
swamping  effects  of  intercrossing,  and  consequent  impor- 
tance of  isolation.  But  it  makes  against  physiological 
selection  by  showing  that  the  geographical  form  of  isolation 
is  sufficient  to  explain  all  the  cases  of  specific  differentiation 
in  birds.  But  I  must  remember  that  the  latter  point  rests 
largely  on  negative  inference,  and  that  birds,  owing  to 
their  highly  locomotive  habits,  are  the  class  of  animals  where 
physiological  selection  is  likely  to  be  most  handicapped. 

Herbert  on  Hybridization. — Herbert  tells  us  that  when  he 
first  astonished  the  Horticultural  Society  by  laying  before  them 
the  results  of  his  experiments  on  hybridization,  his  brother 
botanists  took  serious  alarm.  For  it  appeared  to  them  that 
this  "intermixture  of  species  would  confuse  the  labours 
of  botanists,  and  force  them  to  work  their  way  through 
a  wilderness  of  uncertainty."  Therefore  he  was  bluntly  told 


174         Darwin,  and  after  Darwin. 

by  several  of  these  gentlemen,  "  I  do  not  thank  you  for  your 
mules."  Now,  although  naturalists  have  travelled  far  and 
learnt  much  since  those  days,  it  appears  to  me  that  a  modern 
evolutionist  might  still  turn  to  the  horticulturist  with  the  same 
words.  For  assuredly  he  has  no  reason  to  thank  the 
horticulturist  for  his  mules,  until  he  has  found  a  satisfactory 
answer  to  the  question  why  it  is  that  natural  species  differ  so 
profoundly  as  regards  their  capacity  for  hybridizing. 

Advance  on  Herbert's  Position. — If  it  be  said  that  all  my 
work  amounts  to  showing  what  Herbert  said  long  ago — viz. 
that  the  only  true  or  natural  distinction  between  organic  types 
is  the  sexual  distinction — I  answer  that  my  work  does  much 
more  than  this.  For  it  shows  that  the  principle  of  sterility 
is  the  main  condition  to  the  differentiation,  not  merely  of 
species  and  genera,  but  also  to  the  evolution  of  adaptations 
everywhere,  in  higher  as  well  as  in  lower  taxonomic  divisions. 
Moreover,  even  though  naturalists  were  everywhere  to  consent 
to  abandon  specific  designations,  and,  as  Herbert  advises,  to 
"entrench  themselves  behind  genera,"  there  would  still  re- 
main the  facts  of  what  are  now  called  specific  differences  (of 
the  secondary  or  morphological  kind),  and  by  whatever  name 
these  are  called,  they  alike  demand  explanation  at  the  hands 
of  the  evolutionist. 

Fritz  Miiller  on  Cross-infertility. — Fritz  Miiller  writes, 
"  Every  plant  requires,  for  the  production  of  the  strongest 
possible  and  most  prolific  progeny,  a  certain  amount  of 
difference  between  male  and  female  elements  which  unite. 
Fertility  is  diminished  as  well  when  this  degree  is  too  low 
(in  relatives  too  closely  allied)  as  when  it  is  too  high  (in 
those  too  little  related)."  Then  he  adds,  as  a  general  rule, 
"  Species  which  are  wholly  sterile  with  pollen  of  the  same 
stock,  and  even  with  pollen  of  nearly  allied  stocks,  will 
generally  be  fertilized  very  readily  by  the  pollen  of  another 
species.  The  self-sterile  species  of  the  genus  Abulilon, 


Appendix  C.  175 

which  are,  on  the  other  hand,  so  much  inclined  to  hybri- 
dization, afford  a  good  example  of  this  theory,  which  appears 
to  be  confirmed  also  by  Lobelia,  Passiflora,  and  Oncidium  " 
(American  Naturalist,  vol.  viii,  pp.  223—4,  1874). 

Different  groups  of  plants  exhibit  remarkable  differences  in 
the  capability  of  their  constituent  species  to  hybridize. — In  so 
far  as  these  differences  have  reference  only  to  first  crosses, 
they  have  no  bearing  either  for  or  against  my  theory.  Only 
in  so  far  as  the  differences  extend  to  the  production  of  fertile 
hybrids  does  any  question  arise  for  me.  First  of  all,  therefore, 
I  must  ascertain  whether  (or  how  far)  there  is  any  correlation 
between  groups  whose  species  manifest  aptitude  to  form  first 
crosses,  and  groups  where  first  crosses  manifest  aptitude 
to  produce  fertile  hybrids.  Next,  whatever  the  result  of  this 
inquiry  should  be,  if  I  find  that  certain  natural  groups  of 
plants  exhibit  comparatively  well-marked  tendencies  to  form 
fertile  hybrids,  the  question  will  arise,  Are  these  tendencies 
correlated  with  paucity  of  species  ?  If  they  are,  the  fact 
would  make  strongly  in  favour  of  physiological  selection. 
For  the  fact  would  mean  that  in  these  natural  groups,  owing 
to  "  the  nature  of  the  organisms  "  included  under  them,  less 
opportunity  is  given  to  physiological  selection  in  its  work  of 
differentiating  specific  types  than  is  given  by  other  natural 
groups  where  the  nature  of  the  organism  renders  them  more 
prone  to  mutual  sterility.  But  in  prosecuting  this  branch 
of  verification,  I  must  remember  to  allow  for  possibilities  of 
differential  degrees  of  geographical  isolation  in  the  different 
groups  compared. 

On  this  subject  Focke  writes  me  as  follows : — "  In  a 
natural  group  (family,  order,  genus)  showing  considerable 
variability  in  the  structure  of  the  flower,  we  may  expect 
to  find  [or  do  find]  a  greater  number  of  mules  than  in 
a  group  whose  species  are  only  distinguished  by  differ- 
ences in  the  shape  of  the  leaves,  or  in  growth,  &c.  I  do  not 


176        Darwin,  and  after  Darwin. 

know,  however,  which  in  this  connexion  of  things  is  the 
cause  and  which  the  effect.  A  useful  ancestral  structure  of 
the  flower  may  be  conserved  by  an  otherwise  varying  pro- 
geny, on  condition  that  the  progress  of  diversity  be  not 
disturbed  by  frequent  intercrossings.  [Therefore,  if  this 
condition  be  satisfied,  the  structure  of  the  flower  in  different 
members  of  the  group  will  continue  constant :  here  the  cause 
of  constancy  in  the  flower  (however  much  variability  there 
may  be  in  the  leaves,  &c.)  is  its  original  inability  to  hybri- 
dize.] On  the  other  hand,  in  species  or  groups  ready  to 
hybridize  [or  capable  of  hybridizing],  the  fixation  of  a  new 
specific  type  will  require  some  change  in  the  structure  of  the 
flower,  and  a  change  considerable  enough  to  alter  the  con- 
ditions of  fertilization.  [Here  the  reason  of  the  z'«constancy 
of  the  flower  in  different  members  of  the  group  is  the 
original  aptitude  of  their  ancestral  forms  to  hybridize.] 
Perhaps  there  is  something  in  this  suggestion,  but  certainly 
there  are  other  efficient  physiological  relations,  which  are 
at  present  unknown.  Your  theory  of  physiological  selection 
may  serve  to  explain  many  difficult  facts." 

The  Importance  of  Prepotency. — A.  Kerner  shows  by  means 
of  his  own  observations  on  sundry  species  of  plants  which 
hybridize  in  the  wild  state,  that  they  do  so  very  much  more 
frequently  if  both,  or  even  if  only  one  of  the  parent  forms  be 
rare  in  the  neighbourhood.  This  fact  can  only  be  explained 
by  supposing  that,  even  in  species  most  prone  to  hybridizing 
under  Nature,  there  is  some  degree  of  prepotency  of  pollen 
of  the  same  species  over  that  of  the  other  species ;  so  that 
where  both  species  are  common,  it  is  correspondingly  rare 
that  the  foreign  pollen  gets  a  chance.  But  if  there  were  no 
prepotency,  the  two  species  would  blend ;  and  this  Kerner 
supposes  must  actually  take  place  wherever  two  previously 
separated  species,  thus  physiologically  circumstanced,  happen 
to  be  brought  together.  (Kerner's  paper  is  published  in 


Appendix  C.  177 

Oesler.  Bot.  Zeitschrift,  XXI,  1871,  where  he  alludes  to 
sundry  other  papers  of  his  own  advocating  similar  views.) 

The  relation  of  these  observations  to  Jordan's  especes  affinti 
is  obvious.  We  have  only  to  suppose  that  some  such  slight 
and  constant  difference  characterizes  the  sexual  elements  of 
these  allied  varieties  as  demonstrably  characterizes  their 
morphology,  and  we  can  understand  how  pollen-prepotency 
would  keep  the  forms  distinct — such  forms,  therefore,  being 
so  many  records  of  such  prepotency. 

Both  from  Kerner's  work,  and  still  more  from  that  of 
Jordan  and  Nageli,  I  conclude  that  (at  all  events  in  plants) 
prepotency  is  the  way  in  which  physiological  selection 
chiefly  acts.  That  is  to  say,  sudden  and  extreme  variations  in 
the  way  of  sexual  incompatibility  are  probably  rare,  as  com- 
pared with  some  degree  of  prepotency.  According  as  this 
degree  is  small  or  great  so  will  be  the  amount  of  the 
corresponding  separation.  This  view  would  show  that  in 
plants  the  principle  of  physiological  selection  is  one  of 
immensely  widespread  influence,  causing  (on  the  same 
areas)  more  or  less  permanent  varieties  much  below  specific 
rank.  And  when  we  remember  on  how  delicate  a  balance 
of  physiological  conditions  complete  correspondency  of  pollen 
to  ovules  depends,  we  may  be  prepared  to  expect  that  the 
phenomenon  of  prepotency  is  not  of  uncommon  occurrence. 

Self-fertilization  and  Variability. — It  occurred  to  Count 
Berg  Sagnitz  that,  if  physiological  selection  is  a  true 
principle  in  nature,  vegetable  species  in  which  self- 
fertilization  obtains  ought  to  be  more  rich  in  constant 
varieties  than  are  species  in  which  cross-fertilization  rules. 
For,  although  even  in  the  latter  case  physiological  isolation 
may  occasionally  arise,  it  cannot  be  of  such  habitual  or 
constant  occurrence  as  it  must  be  in  the  former  case. 
Acting  on  this  idea,  Count  Berg  Sagnitz  applied  himself  to 
ascertain  whether  there  is  any  general  correlation  between  the 

III.  N 


178        Darwin,  and  after  Darwin. 

habit  of  self-fertilization  and  the  fact  of  high  variability  ;  and 
he  says  that  in  all  the  cases  which  he  has  hitherto  investi- 
gated, the  correlation  in  question  is  unmistakable. 

Additional  Hypothesis  concerning  Physiological  Selection. — 
In  reciprocal  crosses  A  x  B  is  often  more  fertile  than 
B  x  A.  If  hybrid  AB  is  more  fertile  with  A,  and  hybrid 
BA  with  B,  than  vice  versa,  there  would  be  given  a  good 
analogy  on  which  to  found  the  following  hypothesis. 

Let  A  and  B  be  two  intergenerating  groups  in  which 
segregate  fecundity  is  first  beginning.  Of  the  hybrids,  AB 
will  be  more  fertile  with  A,  and  BA  with  B,  than  vice  versa. 
The  interbreeding  of  AB  with  A  will  eventually  modify 
sexual  characters  of  A  by  assimilating  it  to  those  of  ABy 
while  the  interbreeding  of  BA  with  B  will  similarly  modify 
sexual  characters  of  B  by  assimilating  it  to  those  of  BA. 
Consequently,  A  will  become  more  and  more  infertile  with 
£,  while  B  becomes  more  and  more  infertile  with  A.  Fewer 
and  fewer  hybrids  will  thus  be  produced  till  mutual  sterility 
is  complete. 

To  sustain  this  hypothesis  it  would  be  needful  to  prove 
experimentally,  (i)  that  hybrid  forms  AB  are  more  fertile 
with  A  than  with  B,  while  hybrid  forms  BA  are  more  fertile 
with  B  than  with  A  [or,  it  may  be  possible  that  the  opposite 
relations  would  be  found  to  obtain,  viz.  that  A  B  would  be 
more  fertile  with  B,  and  BA  with  A] ;  (2)  that,  if  so, 
effect  of  intercrossing  AB  with  A  is  to  make  progeny  more 
fertile  with  A  than  with  £,  while  effect  of  intercrossing  BA 
with  B  is  to  make  progeny  more  fertile  with  B  than  with  A. 

Such  experiments  had  best  be  tried  with  species  where 
there  is  already  known  to  be  a  difference  of  fertility  between 
reciprocal  crosses  (e.g.  Matthiola  annua  and  M.  glabra,  see 
Origin  of  Species >  p.  244). 


INDEX 


ALLEN,  Mr.  J.  A.,  on  variation 

under  nature,  34. 
Amixia,    12-28,    110-115,    117- 

133- 
Apogamy,  5,  6,  10,  18,  28. 


BELT,  on  physiological  selection, 

44- 
BERG   SAGNITZ,  Count,  on  self- 

fertilization  and  variability,  177. 
Breeding,  separate  and  segregate, 

Butterflies  of  polar  regions  and 
Alps,  133. 

C. 

CATCHPOOL,  Mr.,  on  physiological 
selection.  44,  137. 

Cross-infertility,  46;  and  varietal 
divergence,  82  ;  and  diversity  of 
life,  169;  and  stability,  170; 
and  specific  differentiation,  170; 
in  domesticated  cattle,  170; 
testing  for,  172;  Fritz  Muller  on, 
174. 

D. 

DARWIN,  Charles,  on  isolation. 
2,  106  ;  on  diversity  under 
nature,  31  ;  on  the  fertility  of 
varieties,  50;  on  the  origin  oi 
cross-infertility,  51;  on  distri- 
bution, 68  ;  on  prepotency.  89  ; 
on  geographical  isolation,  101, 
108;  on  methodical  selection. 

N 


1 02  ;  on  modification  in  large 
areas,  103  ;  on  the  swamping 
effects  of  intercrossing,  105  ;  on 
independent  variability,  109  ;  on 
domestic  animals,  no. 

DELBCEUF,  law  of  independent 
variability,  13. 

Differentiation  under  natural  se- 
lection, 37. 

Diversity  of  life  and  cross-infer- 
tility, 169. 

Domesticated  cattle  and  cross- 
i;  fertility,  170,  172. 

E. 

Evidences  of  physiological  selec- 
tion, 62. 

Evolution,  monotypic  and  poly- 
typic,  21,  75,  102,  107,  in, 
129. 

Experimental  research  in  physio- 
logical selection,  85. 

F. 

Fertility  of  domesticated  varieties, 

172. 
FocKE,    Herr,    on    hybridization, 

»75- 

G. 

GALTON,    Mr.    Francis,    law    of 

regression,  39. 
General  conclusions,  144. 
Geographical      distribution     and 

physiological  selection,  65. 
GIARD,  M.,  on  apogarm  .  .4. 
GRABHAM,  Dr.,  on  mollusca  of 

Madeira,  135. 

2 


i8o 


Index. 


GULICK,  Rev.  J.,  on  natural 
selection  as  a  mode  of  isolation, 
9 ;  on  divergence,  1 1  ;  on  segre- 
gate breeding,  19;  on  geogra- 
phical distribution,  27 ;  on  the 
prevention  of  intercrossing,  127; 
on  Mr.  Wallace's  criticisms,  151. 

H. 

HERBERT,  on  hybridization,  173; 

advance  on  his  position,  174. 
HERDMAN,  Prof.,  on  physiological 

isolation,  123. 
Historical  sketch  of  opinions  on 

isolation,  101. 
Homogamy,  5,  6  ;  forms  of,  7,  19, 

29. 
Hybridization,  HERBERT  on,  173 ; 

in  plants,  175. 
Hypothesis,  additional,  concerning 

physiological  selection,  178. 


Independent  variability,  1 2-29. 

Isolation,  defined,  2  ;  forms  of. 
3,  6 ;  geographical,  3 ;  discri- 
minate and  indiscriminate,  5 ; 
physiological,  9,  41,  58;  its 
importance,  39  ;  sketch  of 
opinions  on,  101 ;  general  con- 
clusions, 144;  SEEBOHM  on,  173. 


JORDAN,  M.,  on  cross  sterile 
varieties  of  plants,  86;  his  re- 
searches summarized,  87. 

K. 

KERNER,  Prof.  A.,  on  prepotency, 
I76. 

L. 

LANKESTER,  Prof.  Ray,  on  di- 
vergent evolution,  15. 

LE  CONTE,  Prof.,  on  fossil  snails 
of  Steinheim,  95  ;  on  isolation, 
129. 

LIVINGSTONE,  Dr.  David,  quoted, 
123. 


MELDOLA,  Prof.,  on  difficulty 
from  intercrossing,  lai. 

Misunderstandings  of  physio- 
logical selection,  59. 

Monotypic  evolution,  see  Evolu- 
tion. 

MORGAN,  Prof.  Lloyd,  on  steri- 
lity, 56;  on  isolation,  128. 

MOULTON,  Mr.  Fletcher,  an 
examination  of  Mr.  Wallace's 
calculations  on  physiological 
selection,  157. 

MuLLER,Fritz,on  cross-infertility, 
174. 

N. 

NAGELI,  on  isolation,  76;  on 
synoicy,  78,  82. 

Natural  selection,  a  form  of  dis- 
criminate isolation,  9,  10,  23; 
leads  to  monotypic  evolution, 
24-29  ;  difficulties  of,  41,  51. 

P. 

Panmixia,  12. 

Physiological  selection,  9,  41 ; 
summarized,  58;  misunderstand- 
ings of,  59 ;  evidences  of,  81— 
119;  and  Weismannism,  169; 
additional  hypothesis,  178. 

Polytypic  evolution,  see  Evolution. 

Prepotency,  89 ;  importance  of, 
176. 


SCHMIDT,  Prof.  Oscar,  on  do- 
mesticated cattle,  171. 

SEEBOHM  on  isolation,  173. 

Segregation,  28. 

Selection,  physiological,  see  Physi- 
ological selection. 

Self-fertilization  and  variability, 
177. 

Snails  of  Sandwich  Islands,  16, 
130;  fossil  of  Steinheim,  95. 

Specific  differentiation  and  cross- 
infertility,  I 70. 

Stability  and  cross-infertility.  170. 

Synoicy,  78. 


Index. 


181 


T. 

Topographical  distribution  and 
physiological  selection,  74;  of 
varieties,  81. 

Transformation,  serial  and  di- 
vergent, 21,  lai. 

V. 

Variability  and  self-fertilization, 
177- 

Variation  in  birds,  34. 

Varieties,  topographical  distribu- 
tion of,  81. 


W. 

WAGNER,  Maritz,  3;  on  geo- 
graphical isolation,  76 ;  quoted, 
103  ;  law  of  migration,  i  n. 

WALLACE,  Mr.  A.  R.,  3,  17; 
9110:66,34,47,51,57,130-136; 
criticized  by  Gulick,  153. 

WEISMANN,  Prof.,  on  geographical 
isolation,  76,  114-118. 

Weismannism  and  physiological 
selection,  169. 


R.   R.  DONNELLEY  &  SONS    CO. 


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